diff --git a/packages/ast-spec/src/expression/BinaryExpression/fixtures/_error_/private-lhs-not-in/fixture.ts b/packages/ast-spec/src/expression/BinaryExpression/fixtures/_error_/private-lhs-not-in/fixture.ts
new file mode 100644
index 000000000000..daafa9ccb62f
--- /dev/null
+++ b/packages/ast-spec/src/expression/BinaryExpression/fixtures/_error_/private-lhs-not-in/fixture.ts
@@ -0,0 +1,4 @@
+class A {
+  #a;
+  b = #a + 1;
+}
diff --git a/packages/ast-spec/src/expression/BinaryExpression/fixtures/_error_/private-lhs-not-in/snapshots/1-TSESTree-Error.shot b/packages/ast-spec/src/expression/BinaryExpression/fixtures/_error_/private-lhs-not-in/snapshots/1-TSESTree-Error.shot
new file mode 100644
index 000000000000..ec75deb33cff
--- /dev/null
+++ b/packages/ast-spec/src/expression/BinaryExpression/fixtures/_error_/private-lhs-not-in/snapshots/1-TSESTree-Error.shot
@@ -0,0 +1,10 @@
+// Vitest Snapshot v1, https://vitest.dev/guide/snapshot.html
+
+exports[`AST Fixtures > expression > BinaryExpression > _error_ > private-lhs-not-in > TSESTree - Error`]
+TSError
+  1 | class A {
+  2 |   #a;
+> 3 |   b = #a + 1;
+    |       ^^ Private identifiers cannot appear on the right-hand-side of an 'in' expression.
+  4 | }
+  5 |
diff --git a/packages/ast-spec/src/expression/BinaryExpression/fixtures/_error_/private-lhs-not-in/snapshots/2-Babel-Error.shot b/packages/ast-spec/src/expression/BinaryExpression/fixtures/_error_/private-lhs-not-in/snapshots/2-Babel-Error.shot
new file mode 100644
index 000000000000..453c15f5a66d
--- /dev/null
+++ b/packages/ast-spec/src/expression/BinaryExpression/fixtures/_error_/private-lhs-not-in/snapshots/2-Babel-Error.shot
@@ -0,0 +1,11 @@
+// Vitest Snapshot v1, https://vitest.dev/guide/snapshot.html
+
+exports[`AST Fixtures > expression > BinaryExpression > _error_ > private-lhs-not-in > Babel - Error`]
+BabelError
+  1 | class A {
+  2 |   #a;
+> 3 |   b = #a + 1;
+    |       ^ Private names are only allowed in property accesses (`obj.#a`) or in `in` expressions (`#a in obj`). (3:6)
+  4 | }
+  5 |
+  
diff --git a/packages/ast-spec/src/expression/BinaryExpression/fixtures/_error_/private-lhs-not-in/snapshots/3-Alignment-Error.shot b/packages/ast-spec/src/expression/BinaryExpression/fixtures/_error_/private-lhs-not-in/snapshots/3-Alignment-Error.shot
new file mode 100644
index 000000000000..c95813948827
--- /dev/null
+++ b/packages/ast-spec/src/expression/BinaryExpression/fixtures/_error_/private-lhs-not-in/snapshots/3-Alignment-Error.shot
@@ -0,0 +1,4 @@
+// Vitest Snapshot v1, https://vitest.dev/guide/snapshot.html
+
+exports[`AST Fixtures > expression > BinaryExpression > _error_ > private-lhs-not-in > Error Alignment`]
+Both errored
diff --git a/packages/ast-spec/src/expression/BinaryExpression/fixtures/_error_/private-rhs/fixture.ts b/packages/ast-spec/src/expression/BinaryExpression/fixtures/_error_/private-rhs/fixture.ts
new file mode 100644
index 000000000000..ce0e507a49fd
--- /dev/null
+++ b/packages/ast-spec/src/expression/BinaryExpression/fixtures/_error_/private-rhs/fixture.ts
@@ -0,0 +1,4 @@
+class A {
+  #a;
+  b = 1 in #a;
+}
diff --git a/packages/ast-spec/src/expression/BinaryExpression/fixtures/_error_/private-rhs/snapshots/1-TSESTree-Error.shot b/packages/ast-spec/src/expression/BinaryExpression/fixtures/_error_/private-rhs/snapshots/1-TSESTree-Error.shot
new file mode 100644
index 000000000000..67636f0e09ec
--- /dev/null
+++ b/packages/ast-spec/src/expression/BinaryExpression/fixtures/_error_/private-rhs/snapshots/1-TSESTree-Error.shot
@@ -0,0 +1,10 @@
+// Vitest Snapshot v1, https://vitest.dev/guide/snapshot.html
+
+exports[`AST Fixtures > expression > BinaryExpression > _error_ > private-rhs > TSESTree - Error`]
+TSError
+  1 | class A {
+  2 |   #a;
+> 3 |   b = 1 in #a;
+    |            ^^ Private identifiers are only allowed on the left-hand-side of an 'in' expression.
+  4 | }
+  5 |
diff --git a/packages/ast-spec/src/expression/BinaryExpression/fixtures/_error_/private-rhs/snapshots/2-Babel-Error.shot b/packages/ast-spec/src/expression/BinaryExpression/fixtures/_error_/private-rhs/snapshots/2-Babel-Error.shot
new file mode 100644
index 000000000000..2014af3eb868
--- /dev/null
+++ b/packages/ast-spec/src/expression/BinaryExpression/fixtures/_error_/private-rhs/snapshots/2-Babel-Error.shot
@@ -0,0 +1,11 @@
+// Vitest Snapshot v1, https://vitest.dev/guide/snapshot.html
+
+exports[`AST Fixtures > expression > BinaryExpression > _error_ > private-rhs > Babel - Error`]
+BabelError
+  1 | class A {
+  2 |   #a;
+> 3 |   b = 1 in #a;
+    |            ^ Private names are only allowed in property accesses (`obj.#a`) or in `in` expressions (`#a in obj`). (3:11)
+  4 | }
+  5 |
+  
diff --git a/packages/ast-spec/src/expression/BinaryExpression/fixtures/_error_/private-rhs/snapshots/3-Alignment-Error.shot b/packages/ast-spec/src/expression/BinaryExpression/fixtures/_error_/private-rhs/snapshots/3-Alignment-Error.shot
new file mode 100644
index 000000000000..802be146cc95
--- /dev/null
+++ b/packages/ast-spec/src/expression/BinaryExpression/fixtures/_error_/private-rhs/snapshots/3-Alignment-Error.shot
@@ -0,0 +1,4 @@
+// Vitest Snapshot v1, https://vitest.dev/guide/snapshot.html
+
+exports[`AST Fixtures > expression > BinaryExpression > _error_ > private-rhs > Error Alignment`]
+Both errored
diff --git a/packages/ast-spec/src/expression/BinaryExpression/fixtures/private-lhs-in/fixture.ts b/packages/ast-spec/src/expression/BinaryExpression/fixtures/private-lhs-in/fixture.ts
new file mode 100644
index 000000000000..f17ec7e20d07
--- /dev/null
+++ b/packages/ast-spec/src/expression/BinaryExpression/fixtures/private-lhs-in/fixture.ts
@@ -0,0 +1,4 @@
+class A {
+  #a;
+  b = #a in A;
+}
diff --git a/packages/ast-spec/src/expression/BinaryExpression/fixtures/private-lhs-in/snapshots/1-TSESTree-AST.shot b/packages/ast-spec/src/expression/BinaryExpression/fixtures/private-lhs-in/snapshots/1-TSESTree-AST.shot
new file mode 100644
index 000000000000..83deb39f58c9
--- /dev/null
+++ b/packages/ast-spec/src/expression/BinaryExpression/fixtures/private-lhs-in/snapshots/1-TSESTree-AST.shot
@@ -0,0 +1,138 @@
+Program {
+  type: "Program",
+  body: [
+    ClassDeclaration {
+      type: "ClassDeclaration",
+      abstract: false,
+      body: ClassBody {
+        type: "ClassBody",
+        body: [
+          PropertyDefinition {
+            type: "PropertyDefinition",
+            computed: false,
+            declare: false,
+            decorators: [],
+            definite: false,
+            key: PrivateIdentifier {
+              type: "PrivateIdentifier",
+              name: "a",
+
+              range: [12, 14],
+              loc: {
+                start: { column: 2, line: 2 },
+                end: { column: 4, line: 2 },
+              },
+            },
+            optional: false,
+            override: false,
+            readonly: false,
+            static: false,
+            value: null,
+
+            range: [12, 15],
+            loc: {
+              start: { column: 2, line: 2 },
+              end: { column: 5, line: 2 },
+            },
+          },
+          PropertyDefinition {
+            type: "PropertyDefinition",
+            computed: false,
+            declare: false,
+            decorators: [],
+            definite: false,
+            key: Identifier {
+              type: "Identifier",
+              decorators: [],
+              name: "b",
+              optional: false,
+
+              range: [18, 19],
+              loc: {
+                start: { column: 2, line: 3 },
+                end: { column: 3, line: 3 },
+              },
+            },
+            optional: false,
+            override: false,
+            readonly: false,
+            static: false,
+            value: BinaryExpression {
+              type: "BinaryExpression",
+              left: PrivateIdentifier {
+                type: "PrivateIdentifier",
+                name: "a",
+
+                range: [22, 24],
+                loc: {
+                  start: { column: 6, line: 3 },
+                  end: { column: 8, line: 3 },
+                },
+              },
+              operator: "in",
+              right: Identifier {
+                type: "Identifier",
+                decorators: [],
+                name: "A",
+                optional: false,
+
+                range: [28, 29],
+                loc: {
+                  start: { column: 12, line: 3 },
+                  end: { column: 13, line: 3 },
+                },
+              },
+
+              range: [22, 29],
+              loc: {
+                start: { column: 6, line: 3 },
+                end: { column: 13, line: 3 },
+              },
+            },
+
+            range: [18, 30],
+            loc: {
+              start: { column: 2, line: 3 },
+              end: { column: 14, line: 3 },
+            },
+          },
+        ],
+
+        range: [8, 32],
+        loc: {
+          start: { column: 8, line: 1 },
+          end: { column: 1, line: 4 },
+        },
+      },
+      declare: false,
+      decorators: [],
+      id: Identifier {
+        type: "Identifier",
+        decorators: [],
+        name: "A",
+        optional: false,
+
+        range: [6, 7],
+        loc: {
+          start: { column: 6, line: 1 },
+          end: { column: 7, line: 1 },
+        },
+      },
+      implements: [],
+      superClass: null,
+
+      range: [0, 32],
+      loc: {
+        start: { column: 0, line: 1 },
+        end: { column: 1, line: 4 },
+      },
+    },
+  ],
+  sourceType: "script",
+
+  range: [0, 33],
+  loc: {
+    start: { column: 0, line: 1 },
+    end: { column: 0, line: 5 },
+  },
+}
\ No newline at end of file
diff --git a/packages/ast-spec/src/expression/BinaryExpression/fixtures/private-lhs-in/snapshots/2-TSESTree-Tokens.shot b/packages/ast-spec/src/expression/BinaryExpression/fixtures/private-lhs-in/snapshots/2-TSESTree-Tokens.shot
new file mode 100644
index 000000000000..b599e3a8bb83
--- /dev/null
+++ b/packages/ast-spec/src/expression/BinaryExpression/fixtures/private-lhs-in/snapshots/2-TSESTree-Tokens.shot
@@ -0,0 +1,122 @@
+[
+  Keyword {
+    type: "Keyword",
+    value: "class",
+
+    range: [0, 5],
+    loc: {
+      start: { column: 0, line: 1 },
+      end: { column: 5, line: 1 },
+    },
+  },
+  Identifier {
+    type: "Identifier",
+    value: "A",
+
+    range: [6, 7],
+    loc: {
+      start: { column: 6, line: 1 },
+      end: { column: 7, line: 1 },
+    },
+  },
+  Punctuator {
+    type: "Punctuator",
+    value: "{",
+
+    range: [8, 9],
+    loc: {
+      start: { column: 8, line: 1 },
+      end: { column: 9, line: 1 },
+    },
+  },
+  PrivateIdentifier {
+    type: "PrivateIdentifier",
+    value: "a",
+
+    range: [12, 14],
+    loc: {
+      start: { column: 2, line: 2 },
+      end: { column: 4, line: 2 },
+    },
+  },
+  Punctuator {
+    type: "Punctuator",
+    value: ";",
+
+    range: [14, 15],
+    loc: {
+      start: { column: 4, line: 2 },
+      end: { column: 5, line: 2 },
+    },
+  },
+  Identifier {
+    type: "Identifier",
+    value: "b",
+
+    range: [18, 19],
+    loc: {
+      start: { column: 2, line: 3 },
+      end: { column: 3, line: 3 },
+    },
+  },
+  Punctuator {
+    type: "Punctuator",
+    value: "=",
+
+    range: [20, 21],
+    loc: {
+      start: { column: 4, line: 3 },
+      end: { column: 5, line: 3 },
+    },
+  },
+  PrivateIdentifier {
+    type: "PrivateIdentifier",
+    value: "a",
+
+    range: [22, 24],
+    loc: {
+      start: { column: 6, line: 3 },
+      end: { column: 8, line: 3 },
+    },
+  },
+  Keyword {
+    type: "Keyword",
+    value: "in",
+
+    range: [25, 27],
+    loc: {
+      start: { column: 9, line: 3 },
+      end: { column: 11, line: 3 },
+    },
+  },
+  Identifier {
+    type: "Identifier",
+    value: "A",
+
+    range: [28, 29],
+    loc: {
+      start: { column: 12, line: 3 },
+      end: { column: 13, line: 3 },
+    },
+  },
+  Punctuator {
+    type: "Punctuator",
+    value: ";",
+
+    range: [29, 30],
+    loc: {
+      start: { column: 13, line: 3 },
+      end: { column: 14, line: 3 },
+    },
+  },
+  Punctuator {
+    type: "Punctuator",
+    value: "}",
+
+    range: [31, 32],
+    loc: {
+      start: { column: 0, line: 4 },
+      end: { column: 1, line: 4 },
+    },
+  },
+]
\ No newline at end of file
diff --git a/packages/ast-spec/src/expression/BinaryExpression/fixtures/private-lhs-in/snapshots/3-Babel-AST.shot b/packages/ast-spec/src/expression/BinaryExpression/fixtures/private-lhs-in/snapshots/3-Babel-AST.shot
new file mode 100644
index 000000000000..83deb39f58c9
--- /dev/null
+++ b/packages/ast-spec/src/expression/BinaryExpression/fixtures/private-lhs-in/snapshots/3-Babel-AST.shot
@@ -0,0 +1,138 @@
+Program {
+  type: "Program",
+  body: [
+    ClassDeclaration {
+      type: "ClassDeclaration",
+      abstract: false,
+      body: ClassBody {
+        type: "ClassBody",
+        body: [
+          PropertyDefinition {
+            type: "PropertyDefinition",
+            computed: false,
+            declare: false,
+            decorators: [],
+            definite: false,
+            key: PrivateIdentifier {
+              type: "PrivateIdentifier",
+              name: "a",
+
+              range: [12, 14],
+              loc: {
+                start: { column: 2, line: 2 },
+                end: { column: 4, line: 2 },
+              },
+            },
+            optional: false,
+            override: false,
+            readonly: false,
+            static: false,
+            value: null,
+
+            range: [12, 15],
+            loc: {
+              start: { column: 2, line: 2 },
+              end: { column: 5, line: 2 },
+            },
+          },
+          PropertyDefinition {
+            type: "PropertyDefinition",
+            computed: false,
+            declare: false,
+            decorators: [],
+            definite: false,
+            key: Identifier {
+              type: "Identifier",
+              decorators: [],
+              name: "b",
+              optional: false,
+
+              range: [18, 19],
+              loc: {
+                start: { column: 2, line: 3 },
+                end: { column: 3, line: 3 },
+              },
+            },
+            optional: false,
+            override: false,
+            readonly: false,
+            static: false,
+            value: BinaryExpression {
+              type: "BinaryExpression",
+              left: PrivateIdentifier {
+                type: "PrivateIdentifier",
+                name: "a",
+
+                range: [22, 24],
+                loc: {
+                  start: { column: 6, line: 3 },
+                  end: { column: 8, line: 3 },
+                },
+              },
+              operator: "in",
+              right: Identifier {
+                type: "Identifier",
+                decorators: [],
+                name: "A",
+                optional: false,
+
+                range: [28, 29],
+                loc: {
+                  start: { column: 12, line: 3 },
+                  end: { column: 13, line: 3 },
+                },
+              },
+
+              range: [22, 29],
+              loc: {
+                start: { column: 6, line: 3 },
+                end: { column: 13, line: 3 },
+              },
+            },
+
+            range: [18, 30],
+            loc: {
+              start: { column: 2, line: 3 },
+              end: { column: 14, line: 3 },
+            },
+          },
+        ],
+
+        range: [8, 32],
+        loc: {
+          start: { column: 8, line: 1 },
+          end: { column: 1, line: 4 },
+        },
+      },
+      declare: false,
+      decorators: [],
+      id: Identifier {
+        type: "Identifier",
+        decorators: [],
+        name: "A",
+        optional: false,
+
+        range: [6, 7],
+        loc: {
+          start: { column: 6, line: 1 },
+          end: { column: 7, line: 1 },
+        },
+      },
+      implements: [],
+      superClass: null,
+
+      range: [0, 32],
+      loc: {
+        start: { column: 0, line: 1 },
+        end: { column: 1, line: 4 },
+      },
+    },
+  ],
+  sourceType: "script",
+
+  range: [0, 33],
+  loc: {
+    start: { column: 0, line: 1 },
+    end: { column: 0, line: 5 },
+  },
+}
\ No newline at end of file
diff --git a/packages/ast-spec/src/expression/BinaryExpression/fixtures/private-lhs-in/snapshots/4-Babel-Tokens.shot b/packages/ast-spec/src/expression/BinaryExpression/fixtures/private-lhs-in/snapshots/4-Babel-Tokens.shot
new file mode 100644
index 000000000000..b599e3a8bb83
--- /dev/null
+++ b/packages/ast-spec/src/expression/BinaryExpression/fixtures/private-lhs-in/snapshots/4-Babel-Tokens.shot
@@ -0,0 +1,122 @@
+[
+  Keyword {
+    type: "Keyword",
+    value: "class",
+
+    range: [0, 5],
+    loc: {
+      start: { column: 0, line: 1 },
+      end: { column: 5, line: 1 },
+    },
+  },
+  Identifier {
+    type: "Identifier",
+    value: "A",
+
+    range: [6, 7],
+    loc: {
+      start: { column: 6, line: 1 },
+      end: { column: 7, line: 1 },
+    },
+  },
+  Punctuator {
+    type: "Punctuator",
+    value: "{",
+
+    range: [8, 9],
+    loc: {
+      start: { column: 8, line: 1 },
+      end: { column: 9, line: 1 },
+    },
+  },
+  PrivateIdentifier {
+    type: "PrivateIdentifier",
+    value: "a",
+
+    range: [12, 14],
+    loc: {
+      start: { column: 2, line: 2 },
+      end: { column: 4, line: 2 },
+    },
+  },
+  Punctuator {
+    type: "Punctuator",
+    value: ";",
+
+    range: [14, 15],
+    loc: {
+      start: { column: 4, line: 2 },
+      end: { column: 5, line: 2 },
+    },
+  },
+  Identifier {
+    type: "Identifier",
+    value: "b",
+
+    range: [18, 19],
+    loc: {
+      start: { column: 2, line: 3 },
+      end: { column: 3, line: 3 },
+    },
+  },
+  Punctuator {
+    type: "Punctuator",
+    value: "=",
+
+    range: [20, 21],
+    loc: {
+      start: { column: 4, line: 3 },
+      end: { column: 5, line: 3 },
+    },
+  },
+  PrivateIdentifier {
+    type: "PrivateIdentifier",
+    value: "a",
+
+    range: [22, 24],
+    loc: {
+      start: { column: 6, line: 3 },
+      end: { column: 8, line: 3 },
+    },
+  },
+  Keyword {
+    type: "Keyword",
+    value: "in",
+
+    range: [25, 27],
+    loc: {
+      start: { column: 9, line: 3 },
+      end: { column: 11, line: 3 },
+    },
+  },
+  Identifier {
+    type: "Identifier",
+    value: "A",
+
+    range: [28, 29],
+    loc: {
+      start: { column: 12, line: 3 },
+      end: { column: 13, line: 3 },
+    },
+  },
+  Punctuator {
+    type: "Punctuator",
+    value: ";",
+
+    range: [29, 30],
+    loc: {
+      start: { column: 13, line: 3 },
+      end: { column: 14, line: 3 },
+    },
+  },
+  Punctuator {
+    type: "Punctuator",
+    value: "}",
+
+    range: [31, 32],
+    loc: {
+      start: { column: 0, line: 4 },
+      end: { column: 1, line: 4 },
+    },
+  },
+]
\ No newline at end of file
diff --git a/packages/ast-spec/src/expression/BinaryExpression/fixtures/private-lhs-in/snapshots/5-AST-Alignment-AST.shot b/packages/ast-spec/src/expression/BinaryExpression/fixtures/private-lhs-in/snapshots/5-AST-Alignment-AST.shot
new file mode 100644
index 000000000000..be95b85d8e44
--- /dev/null
+++ b/packages/ast-spec/src/expression/BinaryExpression/fixtures/private-lhs-in/snapshots/5-AST-Alignment-AST.shot
@@ -0,0 +1,146 @@
+// Jest Snapshot v1, https://goo.gl/fbAQLP
+
+exports[`AST Fixtures expression BinaryExpression private-lhs-in AST Alignment - AST 1`] = `
+"Snapshot Diff:
+- TSESTree
++ Babel
+
+  Program {
+    type: 'Program',
+    body: Array [
+      ClassDeclaration {
+        type: 'ClassDeclaration',
+-       abstract: false,
+        body: ClassBody {
+          type: 'ClassBody',
+          body: Array [
+            PropertyDefinition {
+              type: 'PropertyDefinition',
+              computed: false,
+-             declare: false,
+-             decorators: Array [],
+-             definite: false,
+              key: PrivateIdentifier {
+                type: 'PrivateIdentifier',
+                name: 'a',
+
+                range: [12, 14],
+                loc: {
+                  start: { column: 2, line: 2 },
+                  end: { column: 4, line: 2 },
+                },
+              },
+-             optional: false,
+-             override: false,
+-             readonly: false,
+              static: false,
+              value: null,
+
+              range: [12, 15],
+              loc: {
+                start: { column: 2, line: 2 },
+                end: { column: 5, line: 2 },
+              },
+            },
+            PropertyDefinition {
+              type: 'PropertyDefinition',
+              computed: false,
+-             declare: false,
+-             decorators: Array [],
+-             definite: false,
+              key: Identifier {
+                type: 'Identifier',
+-               decorators: Array [],
+                name: 'b',
+-               optional: false,
+
+                range: [18, 19],
+                loc: {
+                  start: { column: 2, line: 3 },
+                  end: { column: 3, line: 3 },
+                },
+              },
+-             optional: false,
+-             override: false,
+-             readonly: false,
+              static: false,
+              value: BinaryExpression {
+                type: 'BinaryExpression',
+                left: PrivateIdentifier {
+                  type: 'PrivateIdentifier',
+                  name: 'a',
+
+                  range: [22, 24],
+                  loc: {
+                    start: { column: 6, line: 3 },
+                    end: { column: 8, line: 3 },
+                  },
+                },
+                operator: 'in',
+                right: Identifier {
+                  type: 'Identifier',
+-                 decorators: Array [],
+                  name: 'A',
+-                 optional: false,
+
+                  range: [28, 29],
+                  loc: {
+                    start: { column: 12, line: 3 },
+                    end: { column: 13, line: 3 },
+                  },
+                },
+
+                range: [22, 29],
+                loc: {
+                  start: { column: 6, line: 3 },
+                  end: { column: 13, line: 3 },
+                },
+              },
+
+              range: [18, 30],
+              loc: {
+                start: { column: 2, line: 3 },
+                end: { column: 14, line: 3 },
+              },
+            },
+          ],
+
+          range: [8, 32],
+          loc: {
+            start: { column: 8, line: 1 },
+            end: { column: 1, line: 4 },
+          },
+        },
+-       declare: false,
+-       decorators: Array [],
+        id: Identifier {
+          type: 'Identifier',
+-         decorators: Array [],
+          name: 'A',
+-         optional: false,
+
+          range: [6, 7],
+          loc: {
+            start: { column: 6, line: 1 },
+            end: { column: 7, line: 1 },
+          },
+        },
+-       implements: Array [],
+        superClass: null,
+
+        range: [0, 32],
+        loc: {
+          start: { column: 0, line: 1 },
+          end: { column: 1, line: 4 },
+        },
+      },
+    ],
+    sourceType: 'script',
+
+    range: [0, 33],
+    loc: {
+      start: { column: 0, line: 1 },
+      end: { column: 0, line: 5 },
+    },
+  }"
+`;
diff --git a/packages/ast-spec/src/expression/BinaryExpression/fixtures/private-lhs-in/snapshots/6-AST-Alignment-Tokens.shot b/packages/ast-spec/src/expression/BinaryExpression/fixtures/private-lhs-in/snapshots/6-AST-Alignment-Tokens.shot
new file mode 100644
index 000000000000..31764d700643
--- /dev/null
+++ b/packages/ast-spec/src/expression/BinaryExpression/fixtures/private-lhs-in/snapshots/6-AST-Alignment-Tokens.shot
@@ -0,0 +1,136 @@
+// Jest Snapshot v1, https://goo.gl/fbAQLP
+
+exports[`AST Fixtures expression BinaryExpression private-lhs-in AST Alignment - Token 1`] = `
+"Snapshot Diff:
+- TSESTree
++ Babel
+
+  Array [
+    Keyword {
+      type: 'Keyword',
+      value: 'class',
+
+      range: [0, 5],
+      loc: {
+        start: { column: 0, line: 1 },
+        end: { column: 5, line: 1 },
+      },
+    },
+    Identifier {
+      type: 'Identifier',
+      value: 'A',
+
+      range: [6, 7],
+      loc: {
+        start: { column: 6, line: 1 },
+        end: { column: 7, line: 1 },
+      },
+    },
+    Punctuator {
+      type: 'Punctuator',
+      value: '{',
+
+      range: [8, 9],
+      loc: {
+        start: { column: 8, line: 1 },
+        end: { column: 9, line: 1 },
+      },
+    },
+-   Identifier {
+-     type: 'Identifier',
+-     value: '#a',
++   PrivateIdentifier {
++     type: 'PrivateIdentifier',
++     value: 'a',
+
+      range: [12, 14],
+      loc: {
+        start: { column: 2, line: 2 },
+        end: { column: 4, line: 2 },
+      },
+    },
+    Punctuator {
+      type: 'Punctuator',
+      value: ';',
+
+      range: [14, 15],
+      loc: {
+        start: { column: 4, line: 2 },
+        end: { column: 5, line: 2 },
+      },
+    },
+    Identifier {
+      type: 'Identifier',
+      value: 'b',
+
+      range: [18, 19],
+      loc: {
+        start: { column: 2, line: 3 },
+        end: { column: 3, line: 3 },
+      },
+    },
+    Punctuator {
+      type: 'Punctuator',
+      value: '=',
+
+      range: [20, 21],
+      loc: {
+        start: { column: 4, line: 3 },
+        end: { column: 5, line: 3 },
+      },
+    },
+-   Identifier {
+-     type: 'Identifier',
+-     value: '#a',
++   PrivateIdentifier {
++     type: 'PrivateIdentifier',
++     value: 'a',
+
+      range: [22, 24],
+      loc: {
+        start: { column: 6, line: 3 },
+        end: { column: 8, line: 3 },
+      },
+    },
+    Keyword {
+      type: 'Keyword',
+      value: 'in',
+
+      range: [25, 27],
+      loc: {
+        start: { column: 9, line: 3 },
+        end: { column: 11, line: 3 },
+      },
+    },
+    Identifier {
+      type: 'Identifier',
+      value: 'A',
+
+      range: [28, 29],
+      loc: {
+        start: { column: 12, line: 3 },
+        end: { column: 13, line: 3 },
+      },
+    },
+    Punctuator {
+      type: 'Punctuator',
+      value: ';',
+
+      range: [29, 30],
+      loc: {
+        start: { column: 13, line: 3 },
+        end: { column: 14, line: 3 },
+      },
+    },
+    Punctuator {
+      type: 'Punctuator',
+      value: '}',
+
+      range: [31, 32],
+      loc: {
+        start: { column: 0, line: 4 },
+        end: { column: 1, line: 4 },
+      },
+    },
+  ]"
+`;
diff --git a/packages/ast-spec/src/expression/BinaryExpression/spec.ts b/packages/ast-spec/src/expression/BinaryExpression/spec.ts
index 6c262d443064..874df2c6d586 100644
--- a/packages/ast-spec/src/expression/BinaryExpression/spec.ts
+++ b/packages/ast-spec/src/expression/BinaryExpression/spec.ts
@@ -7,9 +7,18 @@ import type { BinaryOperatorToText } from './BinaryOperatorToText';
 
 export * from './BinaryOperatorToText';
 
-export interface BinaryExpression extends BaseNode {
+export interface PrivateInExpression extends BaseNode {
   type: AST_NODE_TYPES.BinaryExpression;
-  left: Expression | PrivateIdentifier;
+  left: PrivateIdentifier;
+  operator: 'in';
+  right: Expression;
+}
+
+export interface SymmetricBinaryExpression extends BaseNode {
+  type: AST_NODE_TYPES.BinaryExpression;
+  left: Expression;
   operator: ValueOf<BinaryOperatorToText>;
   right: Expression;
 }
+
+export type BinaryExpression = PrivateInExpression | SymmetricBinaryExpression;
diff --git a/packages/ast-spec/src/expression/SequenceExpression/fixtures/_error_/private-id-lhs/fixture.ts b/packages/ast-spec/src/expression/SequenceExpression/fixtures/_error_/private-id-lhs/fixture.ts
new file mode 100644
index 000000000000..402329cc285f
--- /dev/null
+++ b/packages/ast-spec/src/expression/SequenceExpression/fixtures/_error_/private-id-lhs/fixture.ts
@@ -0,0 +1,4 @@
+class A {
+  #a;
+  c = (#a, 1);
+}
diff --git a/packages/ast-spec/src/expression/SequenceExpression/fixtures/_error_/private-id-lhs/snapshots/1-TSESTree-Error.shot b/packages/ast-spec/src/expression/SequenceExpression/fixtures/_error_/private-id-lhs/snapshots/1-TSESTree-Error.shot
new file mode 100644
index 000000000000..ba6e5a7a7e5f
--- /dev/null
+++ b/packages/ast-spec/src/expression/SequenceExpression/fixtures/_error_/private-id-lhs/snapshots/1-TSESTree-Error.shot
@@ -0,0 +1,10 @@
+// Vitest Snapshot v1, https://vitest.dev/guide/snapshot.html
+
+exports[`AST Fixtures > expression > SequenceExpression > _error_ > private-id-lhs > TSESTree - Error`]
+TSError
+  1 | class A {
+  2 |   #a;
+> 3 |   c = (#a, 1);
+    |        ^^ Private identifiers cannot appear on the right-hand-side of an 'in' expression.
+  4 | }
+  5 |
diff --git a/packages/ast-spec/src/expression/SequenceExpression/fixtures/_error_/private-id-lhs/snapshots/2-Babel-Error.shot b/packages/ast-spec/src/expression/SequenceExpression/fixtures/_error_/private-id-lhs/snapshots/2-Babel-Error.shot
new file mode 100644
index 000000000000..9006b349152b
--- /dev/null
+++ b/packages/ast-spec/src/expression/SequenceExpression/fixtures/_error_/private-id-lhs/snapshots/2-Babel-Error.shot
@@ -0,0 +1,11 @@
+// Vitest Snapshot v1, https://vitest.dev/guide/snapshot.html
+
+exports[`AST Fixtures > expression > SequenceExpression > _error_ > private-id-lhs > Babel - Error`]
+BabelError
+  1 | class A {
+  2 |   #a;
+> 3 |   c = (#a, 1);
+    |        ^ Private names are only allowed in property accesses (`obj.#a`) or in `in` expressions (`#a in obj`). (3:7)
+  4 | }
+  5 |
+  
diff --git a/packages/ast-spec/src/expression/SequenceExpression/fixtures/_error_/private-id-lhs/snapshots/3-Alignment-Error.shot b/packages/ast-spec/src/expression/SequenceExpression/fixtures/_error_/private-id-lhs/snapshots/3-Alignment-Error.shot
new file mode 100644
index 000000000000..15de730f25f8
--- /dev/null
+++ b/packages/ast-spec/src/expression/SequenceExpression/fixtures/_error_/private-id-lhs/snapshots/3-Alignment-Error.shot
@@ -0,0 +1,4 @@
+// Vitest Snapshot v1, https://vitest.dev/guide/snapshot.html
+
+exports[`AST Fixtures > expression > SequenceExpression > _error_ > private-id-lhs > Error Alignment`]
+Both errored
diff --git a/packages/ast-spec/src/expression/SequenceExpression/fixtures/_error_/private-id-rhs/fixture.ts b/packages/ast-spec/src/expression/SequenceExpression/fixtures/_error_/private-id-rhs/fixture.ts
new file mode 100644
index 000000000000..9f1df9d73fa4
--- /dev/null
+++ b/packages/ast-spec/src/expression/SequenceExpression/fixtures/_error_/private-id-rhs/fixture.ts
@@ -0,0 +1,4 @@
+class A {
+  #a;
+  c = (1, #a);
+}
diff --git a/packages/ast-spec/src/expression/SequenceExpression/fixtures/_error_/private-id-rhs/snapshots/1-TSESTree-Error.shot b/packages/ast-spec/src/expression/SequenceExpression/fixtures/_error_/private-id-rhs/snapshots/1-TSESTree-Error.shot
new file mode 100644
index 000000000000..da508ef53ff0
--- /dev/null
+++ b/packages/ast-spec/src/expression/SequenceExpression/fixtures/_error_/private-id-rhs/snapshots/1-TSESTree-Error.shot
@@ -0,0 +1,10 @@
+// Vitest Snapshot v1, https://vitest.dev/guide/snapshot.html
+
+exports[`AST Fixtures > expression > SequenceExpression > _error_ > private-id-rhs > TSESTree - Error`]
+TSError
+  1 | class A {
+  2 |   #a;
+> 3 |   c = (1, #a);
+    |           ^^ Private identifiers are only allowed on the left-hand-side of an 'in' expression.
+  4 | }
+  5 |
diff --git a/packages/ast-spec/src/expression/SequenceExpression/fixtures/_error_/private-id-rhs/snapshots/2-Babel-Error.shot b/packages/ast-spec/src/expression/SequenceExpression/fixtures/_error_/private-id-rhs/snapshots/2-Babel-Error.shot
new file mode 100644
index 000000000000..4a6a0b8baf68
--- /dev/null
+++ b/packages/ast-spec/src/expression/SequenceExpression/fixtures/_error_/private-id-rhs/snapshots/2-Babel-Error.shot
@@ -0,0 +1,11 @@
+// Vitest Snapshot v1, https://vitest.dev/guide/snapshot.html
+
+exports[`AST Fixtures > expression > SequenceExpression > _error_ > private-id-rhs > Babel - Error`]
+BabelError
+  1 | class A {
+  2 |   #a;
+> 3 |   c = (1, #a);
+    |           ^ Private names are only allowed in property accesses (`obj.#a`) or in `in` expressions (`#a in obj`). (3:10)
+  4 | }
+  5 |
+  
diff --git a/packages/ast-spec/src/expression/SequenceExpression/fixtures/_error_/private-id-rhs/snapshots/3-Alignment-Error.shot b/packages/ast-spec/src/expression/SequenceExpression/fixtures/_error_/private-id-rhs/snapshots/3-Alignment-Error.shot
new file mode 100644
index 000000000000..aeb556b36b22
--- /dev/null
+++ b/packages/ast-spec/src/expression/SequenceExpression/fixtures/_error_/private-id-rhs/snapshots/3-Alignment-Error.shot
@@ -0,0 +1,4 @@
+// Vitest Snapshot v1, https://vitest.dev/guide/snapshot.html
+
+exports[`AST Fixtures > expression > SequenceExpression > _error_ > private-id-rhs > Error Alignment`]
+Both errored
diff --git a/packages/typescript-estree/src/convert.ts b/packages/typescript-estree/src/convert.ts
index bec26fede610..9e540f6adc87 100644
--- a/packages/typescript-estree/src/convert.ts
+++ b/packages/typescript-estree/src/convert.ts
@@ -2405,6 +2405,21 @@ export class Converter {
       // Binary Operations
 
       case SyntaxKind.BinaryExpression: {
+        if (
+          node.operatorToken.kind !== SyntaxKind.InKeyword &&
+          node.left.kind === SyntaxKind.PrivateIdentifier
+        ) {
+          this.#throwError(
+            node.left,
+            "Private identifiers cannot appear on the right-hand-side of an 'in' expression.",
+          );
+        } else if (node.right.kind === SyntaxKind.PrivateIdentifier) {
+          this.#throwError(
+            node.right,
+            "Private identifiers are only allowed on the left-hand-side of an 'in' expression.",
+          );
+        }
+
         // TypeScript uses BinaryExpression for sequences as well
         if (isComma(node.operatorToken)) {
           const result = this.createNode<TSESTree.SequenceExpression>(node, {