diff --git a/.changeset/shy-wasps-begin.md b/.changeset/shy-wasps-begin.md
new file mode 100644
index 0000000000..6db2290030
--- /dev/null
+++ b/.changeset/shy-wasps-begin.md
@@ -0,0 +1,5 @@
+---
+'@lit-labs/ssr': patch
+---
+
+Fix a memory leak when patching directive constructors for SSR.
diff --git a/packages/labs/ssr/src/lib/render-value.ts b/packages/labs/ssr/src/lib/render-value.ts
index 1b55012d9a..1e636bb0c6 100644
--- a/packages/labs/ssr/src/lib/render-value.ts
+++ b/packages/labs/ssr/src/lib/render-value.ts
@@ -95,6 +95,10 @@ const patchIfDirective = (value: unknown) => {
         }
       );
       patchedDirectiveCache.set(directiveCtor, patchedCtor);
+      // If `patchIfDirective` is called with a previously patched directive,
+      // the patched directive will become patched again unless it's also a key
+      // in the WeakMap. This line prevents a memory leak.
+      patchedDirectiveCache.set(patchedCtor, patchedCtor);
     }
     // This property needs to remain unminified.
     setDirectiveClass(value as DirectiveResult, patchedCtor);