diff --git a/src/polytype-esm.js b/src/polytype-esm.js
index 161ba10..c443dba 100644
--- a/src/polytype-esm.js
+++ b/src/polytype-esm.js
@@ -460,12 +460,20 @@ const createUnionProxy =
 };
 
 const defineGlobally =
-() =>
+undo =>
 {
-    if (globalThis.hasOwnProperty('classes'))
+    if (globalThis.hasOwnProperty('classes') === !undo)
         return false;
-    defineMutableDataProperty(globalThis, 'classes', classes);
-    defineMutableDataProperty(_Object, 'getPrototypeListOf', getPrototypeListOf);
+    if (undo)
+    {
+        delete globalThis.classes;
+        delete _Object.getPrototypeListOf;
+    }
+    else
+    {
+        defineMutableDataProperty(globalThis, 'classes', classes);
+        defineMutableDataProperty(_Object, 'getPrototypeListOf', getPrototypeListOf);
+    }
     return true;
 };
 
diff --git a/src/polytype.d.ts.hbs b/src/polytype.d.ts.hbs
index 19eff28..46dff86 100644
--- a/src/polytype.d.ts.hbs
+++ b/src/polytype.d.ts.hbs
@@ -129,20 +129,28 @@ Polytype.ClusteredConstructor<T>;
 {{> classes q="export function "}}
 
 /**
- * Globally defines `classes` and `Object.getPrototypeListOf`.
+ * Globally defines or undefines `classes` and `Object.getPrototypeListOf`.
  *
- * Calling this function allows using Polytype everywhere inside the current JavaScript realm
- * without imports.
+ * Calling this function without arguments or with a falsy argument allows using Polytype everywhere
+ * inside the current JavaScript realm without imports.
+ * Calling it again with a truthy argument reverts the changes made by the previous call.
  *
  * This function is only available in the module versions of Polytype.
  * For most purposes it is better to import the global version of Polytype directly rather than
  * calling this function.
  *
+ * @param undo
+ *
+ * If this argument is falsy or unspecified, `classes` and `Object.getPrototypeListOf` will be
+ * defined globally.
+ * If this argument is truthy, the globals `classes` and `Object.getPrototypeListOf` will be
+ * deleted.
+ *
  * @returns
  *
- * `true` on success; `false` otherwise.
+ * `true` if any changes were made; `false` otherwise.
  */
-export function defineGlobally(): boolean;
+export function defineGlobally(uninstall?: boolean): boolean;
 
 {{> getPrototypeListOf q="export function "}}
 {{else}}
diff --git a/test/spec-helper.js b/test/spec-helper.js
index 3ab317f..508d090 100644
--- a/test/spec-helper.js
+++ b/test/spec-helper.js
@@ -7,12 +7,13 @@
 {
     function backupGlobals()
     {
-        const globalThisDescriptor = Object.getOwnPropertyDescriptor(globalThis, 'globalThis');
+        const _globalThis = globalThis;
 
         let bindDescriptor;
         let classesDescriptor;
         let fnHasInstanceDescriptor;
         let getPrototypeListOfDescriptor;
+        let globalThisDescriptor;
         let isPrototypeOfDescriptor;
         let objHasInstanceDescriptor;
 
@@ -20,7 +21,8 @@
         (
             () =>
             {
-                classesDescriptor = Object.getOwnPropertyDescriptor(globalThis, 'classes');
+                classesDescriptor = Object.getOwnPropertyDescriptor(_globalThis, 'classes');
+                globalThisDescriptor = Object.getOwnPropertyDescriptor(_globalThis, 'globalThis');
                 fnHasInstanceDescriptor =
                 Object.getOwnPropertyDescriptor(Function, Symbol.hasInstance);
                 bindDescriptor = Object.getOwnPropertyDescriptor(Function.prototype, 'bind');
@@ -37,8 +39,8 @@
         (
             () =>
             {
-                setPropertyDescriptor(globalThis, 'globalThis', globalThisDescriptor);
-                Object.defineProperty(globalThis, 'classes', classesDescriptor);
+                Object.defineProperty(_globalThis, 'classes', classesDescriptor);
+                Object.defineProperty(_globalThis, 'globalThis', globalThisDescriptor);
                 setPropertyDescriptor(Function, Symbol.hasInstance, fnHasInstanceDescriptor);
                 // eslint-disable-next-line no-extend-native
                 Object.defineProperty(Function.prototype, 'bind', bindDescriptor);
diff --git a/test/spec/common/define-globally.spec.js b/test/spec/common/define-globally.spec.js
index db96002..9b07412 100644
--- a/test/spec/common/define-globally.spec.js
+++ b/test/spec/common/define-globally.spec.js
@@ -9,26 +9,69 @@ maybeDescribe
     'defineGlobally',
     () =>
     {
+        let defineGlobally;
+
+        before
+        (async () => { defineGlobally = await loadPolytype(); });
+
+        after
+        (() => { defineGlobally = null; });
+
         backupGlobals();
 
-        it
+        describe
         (
-            'returns true',
-            async () =>
+            'with a falsy argument',
+            () =>
             {
-                const defineGlobally = await loadPolytype();
-                delete globalThis.classes;
-                assert.strictEqual(defineGlobally(), true);
+                it
+                (
+                    'returns true',
+                    () =>
+                    {
+                        delete globalThis.classes;
+                        assert.strictEqual(defineGlobally(), true);
+                        assert.isFunction(globalThis.classes);
+                        assert.isFunction(Object.getPrototypeListOf);
+                    },
+                );
+
+                it
+                (
+                    'returns false',
+                    () =>
+                    {
+                        assert.strictEqual(defineGlobally(), false);
+                    },
+                );
             },
         );
 
-        it
+        describe
         (
-            'returns false',
-            async () =>
+            'with a truthy argument',
+            () =>
             {
-                const defineGlobally = await loadPolytype();
-                assert.strictEqual(defineGlobally(), false);
+                it
+                (
+                    'returns true',
+                    () =>
+                    {
+                        assert.strictEqual(defineGlobally(true), true);
+                        assert.isNotFunction(globalThis.classes);
+                        assert.isNotFunction(Object.getPrototypeListOf);
+                    },
+                );
+
+                it
+                (
+                    'returns false',
+                    () =>
+                    {
+                        delete globalThis.classes;
+                        assert.strictEqual(defineGlobally(true), false);
+                    },
+                );
             },
         );
     },