diff --git a/.changeset/blue-olives-fetch.md b/.changeset/blue-olives-fetch.md
new file mode 100644
index 0000000000..5ad7ead6cb
--- /dev/null
+++ b/.changeset/blue-olives-fetch.md
@@ -0,0 +1,5 @@
+---
+'@lit/reactive-element': patch
+---
+
+Initializers added to subclasses are no longer improperly added to superclass.
diff --git a/packages/reactive-element/src/reactive-element.ts b/packages/reactive-element/src/reactive-element.ts
index 9dd8573f61..c9f964c7ae 100644
--- a/packages/reactive-element/src/reactive-element.ts
+++ b/packages/reactive-element/src/reactive-element.ts
@@ -490,8 +490,8 @@ export abstract class ReactiveElement
    * @nocollapse
    */
   static addInitializer(initializer: Initializer) {
-    this._initializers ??= [];
-    this._initializers.push(initializer);
+    this.finalize();
+    (this._initializers ??= []).push(initializer);
   }
 
   static _initializers?: Initializer[];
@@ -762,6 +762,12 @@ export abstract class ReactiveElement
     // finalize any superclasses
     const superCtor = Object.getPrototypeOf(this) as typeof ReactiveElement;
     superCtor.finalize();
+    // Create own set of initializers for this class if any exist on the
+    // superclass and copy them down. Note, for a small perf boost, avoid
+    // creating initializers unless needed.
+    if (superCtor._initializers !== undefined) {
+      this._initializers = [...superCtor._initializers];
+    }
     this.elementProperties = new Map(superCtor.elementProperties);
     // initialize Map populated in observedAttributes
     this.__attributeToPropertyMap = new Map();
diff --git a/packages/reactive-element/src/test/reactive-element_test.ts b/packages/reactive-element/src/test/reactive-element_test.ts
index 427a40c8dc..82a47a5f23 100644
--- a/packages/reactive-element/src/test/reactive-element_test.ts
+++ b/packages/reactive-element/src/test/reactive-element_test.ts
@@ -2550,28 +2550,55 @@ suite('ReactiveElement', () => {
     assert.equal(a.getAttribute('bar'), 'yo');
   });
 
-  test('addInitializer', () => {
-    class A extends ReactiveElement {
+  suite('initializers', () => {
+    class Base extends ReactiveElement {
       prop1?: string;
       prop2?: string;
       event?: string;
     }
-    A.addInitializer((a) => {
-      (a as A).prop1 = 'prop1';
+    Base.addInitializer((a) => {
+      (a as Base).prop1 = 'prop1';
     });
-    A.addInitializer((a) => {
-      (a as A).prop2 = 'prop2';
+    Base.addInitializer((a) => {
+      (a as Base).prop2 = 'prop2';
     });
-    A.addInitializer((a) => {
-      a.addEventListener('click', (e) => ((a as A).event = e.type));
+    Base.addInitializer((a) => {
+      a.addEventListener('click', (e) => ((a as Base).event = e.type));
+    });
+    customElements.define(generateElementName(), Base);
+
+    test('addInitializer', () => {
+      const a = new Base();
+      container.appendChild(a);
+      assert.equal(a.prop1, 'prop1');
+      assert.equal(a.prop2, 'prop2');
+      a.dispatchEvent(new Event('click'));
+      assert.equal(a.event, 'click');
+    });
+
+    class Sub extends Base {
+      prop3?: string;
+    }
+    Sub.addInitializer((a) => {
+      (a as Sub).prop3 = 'prop3';
+    });
+    customElements.define(generateElementName(), Sub);
+
+    test('addInitializer on subclass', () => {
+      const s = new Sub();
+      container.appendChild(s);
+      assert.equal(s.prop1, 'prop1');
+      assert.equal(s.prop2, 'prop2');
+      assert.equal(s.prop3, 'prop3');
+      s.dispatchEvent(new Event('click'));
+      assert.equal(s.event, 'click');
+    });
+
+    test('addInitializer on subclass independent from superclass', () => {
+      const b = new Base();
+      container.appendChild(b);
+      assert.notOk((b as any).prop3);
     });
-    customElements.define(generateElementName(), A);
-    const a = new A();
-    container.appendChild(a);
-    assert.equal(a.prop1, 'prop1');
-    assert.equal(a.prop2, 'prop2');
-    a.dispatchEvent(new Event('click'));
-    assert.equal(a.event, 'click');
   });
 
   suite('exceptions', () => {