diff --git a/.changeset/loud-hairs-approve.md b/.changeset/loud-hairs-approve.md
new file mode 100644
index 000000000..1ed791aa5
--- /dev/null
+++ b/.changeset/loud-hairs-approve.md
@@ -0,0 +1,5 @@
+---
+'@emotion/weak-memoize': minor
+---
+
+Source code has been migrated to TypeScript. From now on type declarations will be emitted based on that, instead of being hand-written.
diff --git a/packages/memoize/package.json b/packages/memoize/package.json
index 256d93f24..58f5bcedf 100644
--- a/packages/memoize/package.json
+++ b/packages/memoize/package.json
@@ -18,8 +18,7 @@
   },
   "files": [
     "src",
-    "dist",
-    "types/*.d.ts"
+    "dist"
   ],
   "browser": {
     "./dist/emotion-memoize.cjs.js": "./dist/emotion-memoize.browser.cjs.js",
diff --git a/packages/weak-memoize/package.json b/packages/weak-memoize/package.json
index 54c3abc42..0b7786e1a 100644
--- a/packages/weak-memoize/package.json
+++ b/packages/weak-memoize/package.json
@@ -4,7 +4,7 @@
   "description": "A memoization function that uses a WeakMap",
   "main": "dist/emotion-weak-memoize.cjs.js",
   "module": "dist/emotion-weak-memoize.esm.js",
-  "types": "types/index.d.ts",
+  "types": "dist/declarations/src/index.d.ts",
   "license": "MIT",
   "repository": "https://github.com/emotion-js/emotion/tree/main/packages/weak-memoize",
   "scripts": {
@@ -18,8 +18,7 @@
   },
   "files": [
     "src",
-    "dist",
-    "types/*.d.ts"
+    "dist"
   ],
   "browser": {
     "./dist/emotion-weak-memoize.cjs.js": "./dist/emotion-weak-memoize.browser.cjs.js",
diff --git a/packages/weak-memoize/src/index.js b/packages/weak-memoize/src/index.js
deleted file mode 100644
index 0b93c7ee6..000000000
--- a/packages/weak-memoize/src/index.js
+++ /dev/null
@@ -1,15 +0,0 @@
-let weakMemoize = function (
-  /* <Arg, Return> */ func /*: Arg => Return */
-) /*: Arg => Return */ {
-  let cache /*: WeakMap<Arg, Return> */ = new WeakMap()
-  return arg => {
-    if (cache.has(arg)) {
-      return cache.get(arg)
-    }
-    let ret = func(arg)
-    cache.set(arg, ret)
-    return ret
-  }
-}
-
-export default weakMemoize
diff --git a/packages/weak-memoize/src/index.ts b/packages/weak-memoize/src/index.ts
new file mode 100644
index 000000000..ba4c5bc55
--- /dev/null
+++ b/packages/weak-memoize/src/index.ts
@@ -0,0 +1,17 @@
+let weakMemoize = function <Arg extends object, Return>(
+  func: (arg: Arg) => Return
+): (arg: Arg) => Return {
+  let cache = new WeakMap<Arg, Return>()
+  return (arg: Arg) => {
+    if (cache.has(arg)) {
+      // Use non-null assertion because we just checked that the cache `has` it
+      // This allows us to remove `undefined` from the return value
+      return cache.get(arg)!
+    }
+    let ret = func(arg)
+    cache.set(arg, ret)
+    return ret
+  }
+}
+
+export default weakMemoize
diff --git a/packages/weak-memoize/types/index.d.ts b/packages/weak-memoize/types/index.d.ts
index b5344615a..12f6194ac 100644
--- a/packages/weak-memoize/types/index.d.ts
+++ b/packages/weak-memoize/types/index.d.ts
@@ -1,7 +1,3 @@
 // TypeScript Version: 2.2
 
-type UnaryFn<Arg, Return> = (arg: Arg) => Return
-
-export default function weakMemoize<Arg extends object, Return>(
-  func: UnaryFn<Arg, Return>
-): UnaryFn<Arg, Return>
+export { default } from '../src'
diff --git a/packages/weak-memoize/types/tests.ts b/packages/weak-memoize/types/tests.ts
index 8daa3243d..9d54d2849 100644
--- a/packages/weak-memoize/types/tests.ts
+++ b/packages/weak-memoize/types/tests.ts
@@ -1,4 +1,4 @@
-import weakMemoize from '@emotion/weak-memoize'
+import weakMemoize from '../src'
 
 interface Foo {
   bar: 'xyz'
diff --git a/packages/weak-memoize/types/tslint.json b/packages/weak-memoize/types/tslint.json
index c14028519..daf99b517 100644
--- a/packages/weak-memoize/types/tslint.json
+++ b/packages/weak-memoize/types/tslint.json
@@ -18,6 +18,7 @@
     ],
 
     "no-unnecessary-generics": false,
-    "strict-export-declare-modifiers": false
+    "strict-export-declare-modifiers": false,
+    "no-default-import": false
   }
 }