diff --git a/.changeset/beige-coins-type.md b/.changeset/beige-coins-type.md
new file mode 100644
index 0000000000..340170a83e
--- /dev/null
+++ b/.changeset/beige-coins-type.md
@@ -0,0 +1,6 @@
+---
+'@backstage/backend-app-api': patch
+'@backstage/backend-plugin-api': patch
+---
+
+Made `ApiRef.defaultFactory` internal.
diff --git a/packages/backend-app-api/src/wiring/ServiceRegistry.ts b/packages/backend-app-api/src/wiring/ServiceRegistry.ts
index 9df092db42..864390000f 100644
--- a/packages/backend-app-api/src/wiring/ServiceRegistry.ts
+++ b/packages/backend-app-api/src/wiring/ServiceRegistry.ts
@@ -13,12 +13,21 @@
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
+
 import {
   ServiceFactory,
   FactoryFunc,
   ServiceRef,
 } from '@backstage/backend-plugin-api';
 
+/**
+ * Keep in sync with `@backstage/backend-plugin-api/src/services/system/types.ts`
+ * @internal
+ */
+export type InternalServiceRef<T> = ServiceRef<T> & {
+  __defaultFactory?: (service: ServiceRef<T>) => Promise<ServiceFactory<T>>;
+};
+
 export class ServiceRegistry {
   readonly #providedFactories: Map<string, ServiceFactory>;
   readonly #loadedDefaultFactories: Map<Function, Promise<ServiceFactory>>;
@@ -38,7 +47,7 @@ export class ServiceRegistry {
 
   get<T>(ref: ServiceRef<T>): FactoryFunc<T> | undefined {
     let factory = this.#providedFactories.get(ref.id);
-    const { defaultFactory } = ref;
+    const { __defaultFactory: defaultFactory } = ref as InternalServiceRef<T>;
     if (!factory && !defaultFactory) {
       return undefined;
     }
diff --git a/packages/backend-plugin-api/api-report.md b/packages/backend-plugin-api/api-report.md
index 01423390e6..f1e0389191 100644
--- a/packages/backend-plugin-api/api-report.md
+++ b/packages/backend-plugin-api/api-report.md
@@ -180,7 +180,6 @@ export type ServiceFactory<TService = unknown> = {
 export type ServiceRef<T> = {
   id: string;
   T: T;
-  defaultFactory?: (service: ServiceRef<T>) => Promise<ServiceFactory<T>>;
   toString(): string;
   $$ref: 'service';
 };
diff --git a/packages/backend-plugin-api/src/services/system/types.ts b/packages/backend-plugin-api/src/services/system/types.ts
index 3d6ebfc0dd..d622b3917f 100644
--- a/packages/backend-plugin-api/src/services/system/types.ts
+++ b/packages/backend-plugin-api/src/services/system/types.ts
@@ -28,15 +28,20 @@ export type ServiceRef<T> = {
    */
   T: T;
 
+  toString(): string;
+
+  $$ref: 'service';
+};
+
+/**
+ * @internal
+ */
+export type InternalServiceRef<T> = ServiceRef<T> & {
   /**
    * The default factory that will be used to create service
    * instances if no other factory is provided.
    */
-  defaultFactory?: (service: ServiceRef<T>) => Promise<ServiceFactory<T>>;
-
-  toString(): string;
-
-  $$ref: 'service';
+  __defaultFactory?: (service: ServiceRef<T>) => Promise<ServiceFactory<T>>;
 };
 
 /** @public */
@@ -70,12 +75,12 @@ export function createServiceRef<T>(options: {
     get T(): T {
       throw new Error(`tried to read ServiceRef.T of ${this}`);
     },
-    defaultFactory,
     toString() {
       return `serviceRef{${options.id}}`;
     },
     $$ref: 'service', // TODO: declare
-  };
+    __defaultFactory: defaultFactory,
+  } as InternalServiceRef<T>;
 }
 
 /**