diff --git a/packages/core/src/editor/managers/ExtensionManager/ExtensionManager.test.ts b/packages/core/src/editor/managers/ExtensionManager/ExtensionManager.test.ts
index 5e0edadae5..2ec705ab42 100644
--- a/packages/core/src/editor/managers/ExtensionManager/ExtensionManager.test.ts
+++ b/packages/core/src/editor/managers/ExtensionManager/ExtensionManager.test.ts
@@ -334,3 +334,30 @@ describe("ExtensionManager ordering", () => {
     );
   });
 });
+
+describe("ExtensionManager runtime registration", () => {
+  it("installs the plugins of blockNoteExtensions declared by an extension registered at runtime", () => {
+    const subKey = new PluginKey("runtime-sub");
+    const parentKey = new PluginKey("runtime-parent");
+
+    const sub = createExtension(() => ({
+      key: "runtime-sub",
+      prosemirrorPlugins: [new Plugin({ key: subKey })],
+    }));
+    const parent = createExtension(() => ({
+      key: "runtime-parent",
+      blockNoteExtensions: [sub()],
+      prosemirrorPlugins: [new Plugin({ key: parentKey })],
+    }));
+
+    const editor = createMountedEditor([]);
+    editor.registerExtension(parent());
+
+    expect(editor.getExtension(sub)).toBeDefined();
+    expect(pluginIndex(editor, subKey)).not.toBe(-1);
+    // Same order as when the extension is passed to the editor up front.
+    expect(pluginIndex(editor, subKey)).toBeLessThan(
+      pluginIndex(editor, parentKey),
+    );
+  });
+});
diff --git a/packages/core/src/editor/managers/ExtensionManager/index.ts b/packages/core/src/editor/managers/ExtensionManager/index.ts
index 5a3929e8cf..2af8020eeb 100644
--- a/packages/core/src/editor/managers/ExtensionManager/index.ts
+++ b/packages/core/src/editor/managers/ExtensionManager/index.ts
@@ -326,9 +326,15 @@ export class ExtensionManager {
       .concat(toRegister)
       .filter(Boolean) as (Extension | ExtensionFactoryInstance)[];
 
-    const registeredExtensions = newExtensions
-      .map((ext) => this.addExtension(ext))
-      .filter(Boolean) as Extension[];
+    const extensionsBefore = new Set(this.extensions);
+    newExtensions.forEach((ext) => this.addExtension(ext));
+    // `addExtension` also registers the sub-extensions declared in
+    // `blockNoteExtensions`, so take everything it added, in the same order
+    // their plugins get when the editor is created.
+    const getPriority = this.getExtensionPriority();
+    const registeredExtensions = this.extensions
+      .filter((extension) => !extensionsBefore.has(extension))
+      .sort((a, b) => getPriority(b.key) - getPriority(a.key));
 
     const pluginsToAdd: Plugin[] = [];
     for (const extension of registeredExtensions) {
@@ -402,16 +408,11 @@ export class ExtensionManager {
   }
 
   /**
-   * Get all the extensions that are registered to the editor
+   * Returns a function that gives the priority of a registered extension
+   * (higher runs first), based on `runsBefore` and `blockNoteExtensions`.
    */
-  public getTiptapExtensions(): AnyTiptapExtension[] {
-    // Start with the default tiptap extensions
-    const tiptapExtensions = getDefaultTiptapExtensions(
-      this.editor,
-      this.options,
-    ).filter((extension) => !this.disabledExtensions.has(extension.name));
-
-    const getPriority = sortByDependencies(
+  private getExtensionPriority() {
+    return sortByDependencies(
       this.extensions.map((extension) => {
         // A sub-extension declared via `blockNoteExtensions` must run before the
         // extension(s) that declared it, so we merge those parents into its
@@ -426,6 +427,19 @@ export class ExtensionManager {
         };
       }),
     );
+  }
+
+  /**
+   * Get all the extensions that are registered to the editor
+   */
+  public getTiptapExtensions(): AnyTiptapExtension[] {
+    // Start with the default tiptap extensions
+    const tiptapExtensions = getDefaultTiptapExtensions(
+      this.editor,
+      this.options,
+    ).filter((extension) => !this.disabledExtensions.has(extension.name));
+
+    const getPriority = this.getExtensionPriority();
 
     const inputRulesByPriority = new Map<number, InputRule[]>();
     for (const extension of this.extensions) {
diff --git a/packages/core/src/yjs/extensions/index.test.ts b/packages/core/src/yjs/extensions/index.test.ts
new file mode 100644
index 0000000000..9e59f220df
--- /dev/null
+++ b/packages/core/src/yjs/extensions/index.test.ts
@@ -0,0 +1,37 @@
+/**
+ * @vitest-environment jsdom
+ */
+import { describe, expect, it } from "vite-plus/test";
+import { Awareness } from "y-protocols/awareness";
+import * as Y from "yjs";
+
+import { BlockNoteEditor } from "../../editor/BlockNoteEditor.js";
+import { CollaborationExtension } from "./index.js";
+
+describe("CollaborationExtension", () => {
+  it("syncs edits when registered on an existing editor", () => {
+    const doc = new Y.Doc();
+    const fragment = doc.getXmlFragment("doc");
+    const editor = BlockNoteEditor.create({
+      disableExtensions: ["history"],
+      initialContent: [{ type: "paragraph", id: "initialBlockId" }],
+    });
+    editor.mount(document.createElement("div"));
+
+    editor.registerExtension(
+      CollaborationExtension({
+        fragment,
+        user: { name: "Test User", color: "#FF0000" },
+        provider: { awareness: new Awareness(doc) },
+      }),
+    );
+    editor.replaceBlocks(editor.document, [
+      { type: "paragraph", content: "hello" },
+    ]);
+
+    expect(fragment.toJSON()).toContain("hello");
+
+    editor.unmount();
+    doc.destroy();
+  });
+});