diff --git a/.changeset/rare-ladybugs-invite.md b/.changeset/rare-ladybugs-invite.md
new file mode 100644
index 0000000000..4153363b9c
--- /dev/null
+++ b/.changeset/rare-ladybugs-invite.md
@@ -0,0 +1,5 @@
+---
+'@backstage/plugin-auth-backend': patch
+---
+
+Fixed the fallback identity population to correctly generate an entity reference for `userEntityRef` if no token is provided.
diff --git a/plugins/auth-backend/src/lib/oauth/OAuthAdapter.test.ts b/plugins/auth-backend/src/lib/oauth/OAuthAdapter.test.ts
index 92c76b04b7..a3fc77bc02 100644
--- a/plugins/auth-backend/src/lib/oauth/OAuthAdapter.test.ts
+++ b/plugins/auth-backend/src/lib/oauth/OAuthAdapter.test.ts
@@ -17,7 +17,7 @@
 import express from 'express';
 import { THOUSAND_DAYS_MS, TEN_MINUTES_MS, OAuthAdapter } from './OAuthAdapter';
 import { encodeState } from './helpers';
-import { OAuthHandlers } from './types';
+import { OAuthHandlers, OAuthResponse } from './types';
 
 const mockResponseData = {
   providerInfo: {
@@ -36,6 +36,12 @@ const mockResponseData = {
   },
 };
 
+function mkTokenBody(payload: unknown): string {
+  return Buffer.from(JSON.stringify(payload), 'utf8')
+    .toString('base64')
+    .replace(/=/g, '');
+}
+
 describe('OAuthAdapter', () => {
   class MyAuthProvider implements OAuthHandlers {
     async start() {
@@ -249,4 +255,86 @@ describe('OAuthAdapter', () => {
       'Refresh token is not supported for provider test-provider',
     );
   });
+
+  it('correctly populates incomplete identities', async () => {
+    const mockRefresh = jest.fn<Promise<OAuthResponse>, [express.Request]>();
+
+    const oauthProvider = new OAuthAdapter(
+      {
+        refresh: mockRefresh,
+        start: jest.fn(),
+        handler: jest.fn(),
+      } as OAuthHandlers,
+      {
+        ...oAuthProviderOptions,
+        tokenIssuer: {
+          issueToken: async ({ claims }) => `a.${mkTokenBody(claims)}.a`,
+          listPublicKeys: async () => ({ keys: [] }),
+        },
+        disableRefresh: false,
+        isOriginAllowed: () => false,
+      },
+    );
+
+    const mockRequest = {
+      header: () => 'XMLHttpRequest',
+      cookies: {
+        'test-provider-refresh-token': 'token',
+      },
+      query: {},
+    } as unknown as express.Request;
+
+    const mockResponse = {
+      json: jest.fn().mockReturnThis(),
+      status: jest.fn().mockReturnThis(),
+    } as unknown as express.Response;
+
+    // Without a token
+    mockRefresh.mockResolvedValueOnce({
+      ...mockResponseData,
+      backstageIdentity: {
+        id: 'foo',
+        token: '',
+      },
+    });
+    await oauthProvider.refresh(mockRequest, mockResponse);
+    expect(mockResponse.json).toHaveBeenCalledTimes(1);
+    expect(mockResponse.json).toHaveBeenLastCalledWith({
+      ...mockResponseData,
+      backstageIdentity: {
+        id: 'foo',
+        token: `a.${mkTokenBody({ sub: 'user:default/foo' })}.a`,
+        idToken: `a.${mkTokenBody({ sub: 'user:default/foo' })}.a`,
+        identity: {
+          type: 'user',
+          userEntityRef: 'user:default/foo',
+          ownershipEntityRefs: [],
+        },
+      },
+    });
+
+    // With a token
+    mockRefresh.mockResolvedValueOnce({
+      ...mockResponseData,
+      backstageIdentity: {
+        id: 'foo',
+        token: `z.${mkTokenBody({ sub: 'user:my-ns/foo' })}.z`,
+      },
+    });
+    await oauthProvider.refresh(mockRequest, mockResponse);
+    expect(mockResponse.json).toHaveBeenCalledTimes(2);
+    expect(mockResponse.json).toHaveBeenLastCalledWith({
+      ...mockResponseData,
+      backstageIdentity: {
+        id: 'foo',
+        token: `z.${mkTokenBody({ sub: 'user:my-ns/foo' })}.z`,
+        idToken: `z.${mkTokenBody({ sub: 'user:my-ns/foo' })}.z`,
+        identity: {
+          type: 'user',
+          userEntityRef: 'user:my-ns/foo',
+          ownershipEntityRefs: [],
+        },
+      },
+    });
+  });
 });
diff --git a/plugins/auth-backend/src/lib/oauth/OAuthAdapter.ts b/plugins/auth-backend/src/lib/oauth/OAuthAdapter.ts
index eb3f7efa42..4d5d507aa1 100644
--- a/plugins/auth-backend/src/lib/oauth/OAuthAdapter.ts
+++ b/plugins/auth-backend/src/lib/oauth/OAuthAdapter.ts
@@ -17,6 +17,11 @@
 import express from 'express';
 import crypto from 'crypto';
 import { URL } from 'url';
+import {
+  ENTITY_DEFAULT_NAMESPACE,
+  parseEntityRef,
+  stringifyEntityRef,
+} from '@backstage/catalog-model';
 import {
   AuthProviderRouteHandlers,
   AuthProviderConfig,
@@ -243,8 +248,14 @@ export class OAuthAdapter implements AuthProviderRouteHandlers {
       return prepareBackstageIdentityResponse(identity);
     }
 
+    const userEntityRef = stringifyEntityRef(
+      parseEntityRef(identity.id, {
+        defaultKind: 'user',
+        defaultNamespace: ENTITY_DEFAULT_NAMESPACE,
+      }),
+    );
     const token = await this.options.tokenIssuer.issueToken({
-      claims: { sub: identity.id },
+      claims: { sub: userEntityRef },
     });
 
     return prepareBackstageIdentityResponse({ ...identity, token });