diff --git a/packages/eslint-plugin/src/rules/prefer-readonly.ts b/packages/eslint-plugin/src/rules/prefer-readonly.ts
index a393888926cf..ce3f75ebc22d 100644
--- a/packages/eslint-plugin/src/rules/prefer-readonly.ts
+++ b/packages/eslint-plugin/src/rules/prefer-readonly.ts
@@ -173,6 +173,38 @@ export default createRule<Options, MessageIds>({
       };
     }
 
+    function getTypeAnnotationForViolatingNode(
+      node: TSESTree.Node,
+      type: ts.Type,
+      initializerType: ts.Type,
+    ) {
+      const annotation = checker.typeToString(type);
+
+      // verify the about-to-be-added type annotation is in-scope
+      if (tsutils.isTypeFlagSet(initializerType, ts.TypeFlags.EnumLiteral)) {
+        const scope = context.sourceCode.getScope(node);
+        const variable = ASTUtils.findVariable(scope, annotation);
+
+        if (variable == null) {
+          return null;
+        }
+
+        const definition = variable.defs.find(def => def.isTypeDefinition);
+
+        if (definition == null) {
+          return null;
+        }
+
+        const definitionType = services.getTypeAtLocation(definition.node);
+
+        if (definitionType !== type) {
+          return null;
+        }
+      }
+
+      return annotation;
+    }
+
     return {
       [`${functionScopeBoundaries}:exit`](
         node:
@@ -229,13 +261,62 @@ export default createRule<Options, MessageIds>({
             }
           })();
 
+          const typeAnnotation = (() => {
+            if (esNode.type !== AST_NODE_TYPES.PropertyDefinition) {
+              return null;
+            }
+
+            if (esNode.typeAnnotation || !esNode.value) {
+              return null;
+            }
+
+            if (nameNode.type !== AST_NODE_TYPES.Identifier) {
+              return null;
+            }
+
+            const hasConstructorModifications =
+              finalizedClassScope.memberHasConstructorModifications(
+                nameNode.name,
+              );
+
+            if (!hasConstructorModifications) {
+              return null;
+            }
+
+            const violatingType = services.getTypeAtLocation(esNode);
+            const initializerType = services.getTypeAtLocation(esNode.value);
+
+            // if the RHS is a literal, its type would be narrowed, while the
+            // type of the initializer (which isn't `readonly`) would be the
+            // widened type
+            if (initializerType === violatingType) {
+              return null;
+            }
+
+            if (!tsutils.isLiteralType(initializerType)) {
+              return null;
+            }
+
+            return getTypeAnnotationForViolatingNode(
+              esNode,
+              violatingType,
+              initializerType,
+            );
+          })();
+
           context.report({
             ...reportNodeOrLoc,
             messageId: 'preferReadonly',
             data: {
               name: context.sourceCode.getText(nameNode),
             },
-            fix: fixer => fixer.insertTextBefore(nameNode, 'readonly '),
+            *fix(fixer) {
+              yield fixer.insertTextBefore(nameNode, 'readonly ');
+
+              if (typeAnnotation) {
+                yield fixer.insertTextAfter(nameNode, `: ${typeAnnotation}`);
+              }
+            },
           });
         }
       },
@@ -288,6 +369,8 @@ class ClassScope {
   private readonly classType: ts.Type;
   private constructorScopeDepth = OUTSIDE_CONSTRUCTOR;
   private readonly memberVariableModifications = new Set<string>();
+  private readonly memberVariableWithConstructorModifications =
+    new Set<string>();
   private readonly privateModifiableMembers = new Map<
     string,
     ParameterOrPropertyDeclaration
@@ -358,6 +441,7 @@ class ClassScope {
       relationOfModifierTypeToClass === TypeToClassRelation.Instance &&
       this.constructorScopeDepth === DIRECTLY_INSIDE_CONSTRUCTOR
     ) {
+      this.memberVariableWithConstructorModifications.add(node.name.text);
       return;
     }
 
@@ -465,4 +549,8 @@ class ClassScope {
 
     return TypeToClassRelation.Instance;
   }
+
+  public memberHasConstructorModifications(name: string) {
+    return this.memberVariableWithConstructorModifications.has(name);
+  }
 }
diff --git a/packages/eslint-plugin/tests/rules/prefer-readonly.test.ts b/packages/eslint-plugin/tests/rules/prefer-readonly.test.ts
index 1f0d7383c1db..1e2289d65d1e 100644
--- a/packages/eslint-plugin/tests/rules/prefer-readonly.test.ts
+++ b/packages/eslint-plugin/tests/rules/prefer-readonly.test.ts
@@ -956,7 +956,7 @@ class Foo {
       ],
       output: `
         class TestIncorrectlyModifiableDelayed {
-          private readonly incorrectlyModifiableDelayed = 7;
+          private readonly incorrectlyModifiableDelayed: number = 7;
 
           public constructor() {
             this.incorrectlyModifiableDelayed = 7;
@@ -2333,5 +2333,1113 @@ function ClassWithName<TBase extends new (...args: any[]) => {}>(Base: TBase) {
         }
       `,
     },
+    {
+      code: `
+        class Test {
+          private prop = 'hello';
+
+          constructor() {
+            this.prop = 'world';
+          }
+        }
+      `,
+      errors: [
+        {
+          column: 11,
+          data: {
+            name: 'prop',
+          },
+          endColumn: 23,
+          endLine: 3,
+          line: 3,
+          messageId: 'preferReadonly',
+        },
+      ],
+      output: `
+        class Test {
+          private readonly prop: string = 'hello';
+
+          constructor() {
+            this.prop = 'world';
+          }
+        }
+      `,
+    },
+    {
+      code: `
+        class Test {
+          private prop = 'hello';
+        }
+      `,
+      errors: [
+        {
+          column: 11,
+          data: {
+            name: 'prop',
+          },
+          endColumn: 23,
+          endLine: 3,
+          line: 3,
+          messageId: 'preferReadonly',
+        },
+      ],
+      output: `
+        class Test {
+          private readonly prop = 'hello';
+        }
+      `,
+    },
+    {
+      code: `
+        declare const hello: 'hello';
+
+        class Test {
+          private prop = hello;
+
+          constructor() {
+            this.prop = 'world';
+          }
+        }
+      `,
+      errors: [
+        {
+          column: 11,
+          data: {
+            name: 'prop',
+          },
+          endColumn: 23,
+          endLine: 5,
+          line: 5,
+          messageId: 'preferReadonly',
+        },
+      ],
+      output: `
+        declare const hello: 'hello';
+
+        class Test {
+          private readonly prop = hello;
+
+          constructor() {
+            this.prop = 'world';
+          }
+        }
+      `,
+    },
+    {
+      code: `
+        declare const hello: 'hello';
+
+        class Test {
+          private prop = hello;
+        }
+      `,
+      errors: [
+        {
+          column: 11,
+          data: {
+            name: 'prop',
+          },
+          endColumn: 23,
+          endLine: 5,
+          line: 5,
+          messageId: 'preferReadonly',
+        },
+      ],
+      output: `
+        declare const hello: 'hello';
+
+        class Test {
+          private readonly prop = hello;
+        }
+      `,
+    },
+    {
+      code: `
+        class Test {
+          private prop = 10;
+
+          constructor() {
+            this.prop = 11;
+          }
+        }
+      `,
+      errors: [
+        {
+          column: 11,
+          data: {
+            name: 'prop',
+          },
+          endColumn: 23,
+          endLine: 3,
+          line: 3,
+          messageId: 'preferReadonly',
+        },
+      ],
+      output: `
+        class Test {
+          private readonly prop: number = 10;
+
+          constructor() {
+            this.prop = 11;
+          }
+        }
+      `,
+    },
+    {
+      code: `
+        class Test {
+          private prop = 10;
+        }
+      `,
+      errors: [
+        {
+          column: 11,
+          data: {
+            name: 'prop',
+          },
+          endColumn: 23,
+          endLine: 3,
+          line: 3,
+          messageId: 'preferReadonly',
+        },
+      ],
+      output: `
+        class Test {
+          private readonly prop = 10;
+        }
+      `,
+    },
+    {
+      code: `
+        declare const hello: 10;
+
+        class Test {
+          private prop = hello;
+
+          constructor() {
+            this.prop = 11;
+          }
+        }
+      `,
+      errors: [
+        {
+          column: 11,
+          data: {
+            name: 'prop',
+          },
+          endColumn: 23,
+          endLine: 5,
+          line: 5,
+          messageId: 'preferReadonly',
+        },
+      ],
+      output: `
+        declare const hello: 10;
+
+        class Test {
+          private readonly prop = hello;
+
+          constructor() {
+            this.prop = 11;
+          }
+        }
+      `,
+    },
+    {
+      code: `
+        class Test {
+          private prop = true;
+
+          constructor() {
+            this.prop = false;
+          }
+        }
+      `,
+      errors: [
+        {
+          column: 11,
+          data: {
+            name: 'prop',
+          },
+          endColumn: 23,
+          endLine: 3,
+          line: 3,
+          messageId: 'preferReadonly',
+        },
+      ],
+      output: `
+        class Test {
+          private readonly prop: boolean = true;
+
+          constructor() {
+            this.prop = false;
+          }
+        }
+      `,
+    },
+    {
+      code: `
+        class Test {
+          private prop = true;
+        }
+      `,
+      errors: [
+        {
+          column: 11,
+          data: {
+            name: 'prop',
+          },
+          endColumn: 23,
+          endLine: 3,
+          line: 3,
+          messageId: 'preferReadonly',
+        },
+      ],
+      output: `
+        class Test {
+          private readonly prop = true;
+        }
+      `,
+    },
+    {
+      code: `
+        declare const hello: true;
+
+        class Test {
+          private prop = hello;
+
+          constructor() {
+            this.prop = false;
+          }
+        }
+      `,
+      errors: [
+        {
+          column: 11,
+          data: {
+            name: 'prop',
+          },
+          endColumn: 23,
+          endLine: 5,
+          line: 5,
+          messageId: 'preferReadonly',
+        },
+      ],
+      output: `
+        declare const hello: true;
+
+        class Test {
+          private readonly prop = hello;
+
+          constructor() {
+            this.prop = false;
+          }
+        }
+      `,
+    },
+    {
+      code: `
+        enum Foo {
+          Bar,
+          Bazz,
+        }
+
+        class Test {
+          private prop = Foo.Bar;
+
+          constructor() {
+            this.prop = Foo.Bazz;
+          }
+        }
+      `,
+      errors: [
+        {
+          column: 11,
+          data: {
+            name: 'prop',
+          },
+          endColumn: 23,
+          endLine: 8,
+          line: 8,
+          messageId: 'preferReadonly',
+        },
+      ],
+      output: `
+        enum Foo {
+          Bar,
+          Bazz,
+        }
+
+        class Test {
+          private readonly prop: Foo = Foo.Bar;
+
+          constructor() {
+            this.prop = Foo.Bazz;
+          }
+        }
+      `,
+    },
+    {
+      code: `
+        enum Foo {
+          Bar,
+          Bazz,
+        }
+
+        class Test {
+          private prop = Foo.Bar;
+        }
+      `,
+      errors: [
+        {
+          column: 11,
+          data: {
+            name: 'prop',
+          },
+          endColumn: 23,
+          endLine: 8,
+          line: 8,
+          messageId: 'preferReadonly',
+        },
+      ],
+      output: `
+        enum Foo {
+          Bar,
+          Bazz,
+        }
+
+        class Test {
+          private readonly prop = Foo.Bar;
+        }
+      `,
+    },
+    {
+      code: `
+        enum Foo {
+          Bar,
+          Bazz,
+        }
+
+        const foo = Foo.Bar;
+
+        class Test {
+          private prop = foo;
+
+          constructor() {
+            this.prop = foo;
+          }
+        }
+      `,
+      errors: [
+        {
+          column: 11,
+          data: {
+            name: 'prop',
+          },
+          endColumn: 23,
+          endLine: 10,
+          line: 10,
+          messageId: 'preferReadonly',
+        },
+      ],
+      output: `
+        enum Foo {
+          Bar,
+          Bazz,
+        }
+
+        const foo = Foo.Bar;
+
+        class Test {
+          private readonly prop: Foo = foo;
+
+          constructor() {
+            this.prop = foo;
+          }
+        }
+      `,
+    },
+    {
+      code: `
+        enum Foo {
+          Bar,
+          Bazz,
+        }
+
+        const foo = Foo.Bar;
+
+        class Test {
+          private prop = foo;
+        }
+      `,
+      errors: [
+        {
+          column: 11,
+          data: {
+            name: 'prop',
+          },
+          endColumn: 23,
+          endLine: 10,
+          line: 10,
+          messageId: 'preferReadonly',
+        },
+      ],
+      output: `
+        enum Foo {
+          Bar,
+          Bazz,
+        }
+
+        const foo = Foo.Bar;
+
+        class Test {
+          private readonly prop = foo;
+        }
+      `,
+    },
+    {
+      code: `
+        enum Foo {
+          Bar,
+          Bazz,
+        }
+
+        declare const foo: Foo;
+
+        class Test {
+          private prop = foo;
+        }
+      `,
+      errors: [
+        {
+          column: 11,
+          data: {
+            name: 'prop',
+          },
+          endColumn: 23,
+          endLine: 10,
+          line: 10,
+          messageId: 'preferReadonly',
+        },
+      ],
+      output: `
+        enum Foo {
+          Bar,
+          Bazz,
+        }
+
+        declare const foo: Foo;
+
+        class Test {
+          private readonly prop = foo;
+        }
+      `,
+    },
+    {
+      code: `
+        enum Foo {
+          Bar,
+          Bazz,
+        }
+
+        const bar = Foo.Bar;
+
+        function wrapper() {
+          const Foo = 10;
+
+          class Test {
+            private prop = bar;
+
+            constructor() {
+              this.prop = bar;
+            }
+          }
+        }
+      `,
+      errors: [
+        {
+          column: 13,
+          data: {
+            name: 'prop',
+          },
+          endColumn: 25,
+          endLine: 13,
+          line: 13,
+          messageId: 'preferReadonly',
+        },
+      ],
+      output: `
+        enum Foo {
+          Bar,
+          Bazz,
+        }
+
+        const bar = Foo.Bar;
+
+        function wrapper() {
+          const Foo = 10;
+
+          class Test {
+            private readonly prop = bar;
+
+            constructor() {
+              this.prop = bar;
+            }
+          }
+        }
+      `,
+    },
+    {
+      code: `
+        enum Foo {
+          Bar,
+          Bazz,
+        }
+
+        const bar = Foo.Bar;
+
+        function wrapper() {
+          type Foo = 10;
+
+          class Test {
+            private prop = bar;
+
+            constructor() {
+              this.prop = bar;
+            }
+          }
+        }
+      `,
+      errors: [
+        {
+          column: 13,
+          data: {
+            name: 'prop',
+          },
+          endColumn: 25,
+          endLine: 13,
+          line: 13,
+          messageId: 'preferReadonly',
+        },
+      ],
+      output: `
+        enum Foo {
+          Bar,
+          Bazz,
+        }
+
+        const bar = Foo.Bar;
+
+        function wrapper() {
+          type Foo = 10;
+
+          class Test {
+            private readonly prop = bar;
+
+            constructor() {
+              this.prop = bar;
+            }
+          }
+        }
+      `,
+    },
+    {
+      code: `
+        const Bar = (function () {
+          enum Foo {
+            Bar,
+            Bazz,
+          }
+
+          return Foo;
+        })();
+
+        const bar = Bar.Bar;
+
+        class Test {
+          private prop = bar;
+
+          constructor() {
+            this.prop = bar;
+          }
+        }
+      `,
+      errors: [
+        {
+          column: 11,
+          data: {
+            name: 'prop',
+          },
+          endColumn: 23,
+          endLine: 14,
+          line: 14,
+          messageId: 'preferReadonly',
+        },
+      ],
+      output: `
+        const Bar = (function () {
+          enum Foo {
+            Bar,
+            Bazz,
+          }
+
+          return Foo;
+        })();
+
+        const bar = Bar.Bar;
+
+        class Test {
+          private readonly prop = bar;
+
+          constructor() {
+            this.prop = bar;
+          }
+        }
+      `,
+    },
+    {
+      code: `
+        class Test {
+          private prop = { foo: 'bar' };
+        }
+      `,
+      errors: [
+        {
+          column: 11,
+          data: {
+            name: 'prop',
+          },
+          endColumn: 23,
+          endLine: 3,
+          line: 3,
+          messageId: 'preferReadonly',
+        },
+      ],
+      output: `
+        class Test {
+          private readonly prop = { foo: 'bar' };
+        }
+      `,
+    },
+    {
+      code: `
+        class Test {
+          private prop = { foo: 'bar' };
+
+          constructor() {
+            this.prop = { foo: 'bazz' };
+          }
+        }
+      `,
+      errors: [
+        {
+          column: 11,
+          data: {
+            name: 'prop',
+          },
+          endColumn: 23,
+          endLine: 3,
+          line: 3,
+          messageId: 'preferReadonly',
+        },
+      ],
+      output: `
+        class Test {
+          private readonly prop = { foo: 'bar' };
+
+          constructor() {
+            this.prop = { foo: 'bazz' };
+          }
+        }
+      `,
+    },
+    {
+      code: `
+        class Test {
+          private prop = [1, 2, 'three'];
+        }
+      `,
+      errors: [
+        {
+          column: 11,
+          data: {
+            name: 'prop',
+          },
+          endColumn: 23,
+          endLine: 3,
+          line: 3,
+          messageId: 'preferReadonly',
+        },
+      ],
+      output: `
+        class Test {
+          private readonly prop = [1, 2, 'three'];
+        }
+      `,
+    },
+    {
+      code: `
+        class Test {
+          private prop = [1, 2, 'three'];
+
+          constructor() {
+            this.prop = [1, 2, 'four'];
+          }
+        }
+      `,
+      errors: [
+        {
+          column: 11,
+          data: {
+            name: 'prop',
+          },
+          endColumn: 23,
+          endLine: 3,
+          line: 3,
+          messageId: 'preferReadonly',
+        },
+      ],
+      output: `
+        class Test {
+          private readonly prop = [1, 2, 'three'];
+
+          constructor() {
+            this.prop = [1, 2, 'four'];
+          }
+        }
+      `,
+    },
+    {
+      code: `
+        class X {
+          private _isValid = true;
+
+          getIsValid = () => this._isValid;
+
+          constructor(data?: {}) {
+            if (!data) {
+              this._isValid = false;
+            }
+          }
+        }
+      `,
+      errors: [
+        {
+          column: 11,
+          data: {
+            name: '_isValid',
+          },
+          endColumn: 27,
+          endLine: 3,
+          line: 3,
+          messageId: 'preferReadonly',
+        },
+      ],
+      output: `
+        class X {
+          private readonly _isValid: boolean = true;
+
+          getIsValid = () => this._isValid;
+
+          constructor(data?: {}) {
+            if (!data) {
+              this._isValid = false;
+            }
+          }
+        }
+      `,
+    },
+    {
+      code: `
+        class Test {
+          private prop: string = 'hello';
+        }
+      `,
+      errors: [
+        {
+          column: 11,
+          data: {
+            name: 'prop',
+          },
+          endColumn: 23,
+          endLine: 3,
+          line: 3,
+          messageId: 'preferReadonly',
+        },
+      ],
+      output: `
+        class Test {
+          private readonly prop: string = 'hello';
+        }
+      `,
+    },
+    {
+      code: `
+        class Test {
+          private prop: string | number = 'hello';
+        }
+      `,
+      errors: [
+        {
+          column: 11,
+          data: {
+            name: 'prop',
+          },
+          endColumn: 23,
+          endLine: 3,
+          line: 3,
+          messageId: 'preferReadonly',
+        },
+      ],
+      output: `
+        class Test {
+          private readonly prop: string | number = 'hello';
+        }
+      `,
+    },
+    {
+      code: `
+        class Test {
+          private prop: string;
+
+          constructor() {
+            this.prop = 'hello';
+          }
+        }
+      `,
+      errors: [
+        {
+          column: 11,
+          data: {
+            name: 'prop',
+          },
+          endColumn: 23,
+          endLine: 3,
+          line: 3,
+          messageId: 'preferReadonly',
+        },
+      ],
+      output: `
+        class Test {
+          private readonly prop: string;
+
+          constructor() {
+            this.prop = 'hello';
+          }
+        }
+      `,
+    },
+    {
+      code: `
+        class Test {
+          private prop;
+
+          constructor() {
+            this.prop = 'hello';
+          }
+        }
+      `,
+      errors: [
+        {
+          column: 11,
+          data: {
+            name: 'prop',
+          },
+          endColumn: 23,
+          endLine: 3,
+          line: 3,
+          messageId: 'preferReadonly',
+        },
+      ],
+      output: `
+        class Test {
+          private readonly prop;
+
+          constructor() {
+            this.prop = 'hello';
+          }
+        }
+      `,
+    },
+    {
+      code: `
+        class Test {
+          private prop;
+
+          constructor(x: boolean) {
+            if (x) {
+              this.prop = 'hello';
+            } else {
+              this.prop = 10;
+            }
+          }
+        }
+      `,
+      errors: [
+        {
+          column: 11,
+          data: {
+            name: 'prop',
+          },
+          endColumn: 23,
+          endLine: 3,
+          line: 3,
+          messageId: 'preferReadonly',
+        },
+      ],
+      output: `
+        class Test {
+          private readonly prop;
+
+          constructor(x: boolean) {
+            if (x) {
+              this.prop = 'hello';
+            } else {
+              this.prop = 10;
+            }
+          }
+        }
+      `,
+    },
+    {
+      code: `
+        declare const hello: 'hello' | 10;
+
+        class Test {
+          private prop = hello;
+
+          constructor() {
+            this.prop = 10;
+          }
+        }
+      `,
+      errors: [
+        {
+          column: 11,
+          data: {
+            name: 'prop',
+          },
+          endColumn: 23,
+          endLine: 5,
+          line: 5,
+          messageId: 'preferReadonly',
+        },
+      ],
+      output: `
+        declare const hello: 'hello' | 10;
+
+        class Test {
+          private readonly prop = hello;
+
+          constructor() {
+            this.prop = 10;
+          }
+        }
+      `,
+    },
+    {
+      code: `
+        class Test {
+          private prop = null;
+        }
+      `,
+      errors: [
+        {
+          column: 11,
+          data: {
+            name: 'prop',
+          },
+          endColumn: 23,
+          endLine: 3,
+          line: 3,
+          messageId: 'preferReadonly',
+        },
+      ],
+      output: `
+        class Test {
+          private readonly prop = null;
+        }
+      `,
+    },
+    {
+      code: `
+        class Test {
+          private prop = null;
+
+          constructor() {
+            this.prop = null;
+          }
+        }
+      `,
+      errors: [
+        {
+          column: 11,
+          data: {
+            name: 'prop',
+          },
+          endColumn: 23,
+          endLine: 3,
+          line: 3,
+          messageId: 'preferReadonly',
+        },
+      ],
+      output: `
+        class Test {
+          private readonly prop = null;
+
+          constructor() {
+            this.prop = null;
+          }
+        }
+      `,
+    },
+    {
+      code: `
+        class Test {
+          private prop = 'hello' as string;
+        }
+      `,
+      errors: [
+        {
+          column: 11,
+          data: {
+            name: 'prop',
+          },
+          endColumn: 23,
+          endLine: 3,
+          line: 3,
+          messageId: 'preferReadonly',
+        },
+      ],
+      output: `
+        class Test {
+          private readonly prop = 'hello' as string;
+        }
+      `,
+    },
+    {
+      code: `
+        class Test {
+          private prop = Promise.resolve('hello');
+        }
+      `,
+      errors: [
+        {
+          column: 11,
+          data: {
+            name: 'prop',
+          },
+          endColumn: 23,
+          endLine: 3,
+          line: 3,
+          messageId: 'preferReadonly',
+        },
+      ],
+      output: `
+        class Test {
+          private readonly prop = Promise.resolve('hello');
+        }
+      `,
+    },
   ],
 });