diff --git a/.changeset/light-spoons-listen.md b/.changeset/light-spoons-listen.md
new file mode 100644
index 0000000000..ec3a85cb0d
--- /dev/null
+++ b/.changeset/light-spoons-listen.md
@@ -0,0 +1,5 @@
+---
+'@backstage/plugin-auth-backend': patch
+---
+
+Fixed a bug where OAuth state parameters would be serialized as the string `'undefined'`.
diff --git a/plugins/auth-backend/package.json b/plugins/auth-backend/package.json
index 26bd5b408e..38ad491b19 100644
--- a/plugins/auth-backend/package.json
+++ b/plugins/auth-backend/package.json
@@ -50,6 +50,7 @@
     "jose": "^1.27.1",
     "jwt-decode": "^3.1.0",
     "knex": "^0.95.1",
+    "lodash": "^4.17.21",
     "luxon": "^2.0.2",
     "minimatch": "^3.0.3",
     "morgan": "^1.10.0",
diff --git a/plugins/auth-backend/src/lib/oauth/helpers.test.ts b/plugins/auth-backend/src/lib/oauth/helpers.test.ts
index 00161ff8cf..8af1d2f2ad 100644
--- a/plugins/auth-backend/src/lib/oauth/helpers.test.ts
+++ b/plugins/auth-backend/src/lib/oauth/helpers.test.ts
@@ -15,9 +15,39 @@
  */
 
 import express from 'express';
-import { verifyNonce, encodeState } from './helpers';
+import { verifyNonce, encodeState, readState } from './helpers';
 
 describe('OAuthProvider Utils', () => {
+  describe('encodeState', () => {
+    it('should serialized values', () => {
+      const state = {
+        nonce: '123',
+        env: 'development',
+        origin: 'https://example.com',
+      };
+
+      const encoded = encodeState(state);
+      expect(encoded).toBe(
+        Buffer.from(
+          'nonce=123&env=development&origin=https%3A%2F%2Fexample.com',
+        ).toString('hex'),
+      );
+
+      expect(readState(encoded)).toEqual(state);
+    });
+
+    it('should not include undefined values', () => {
+      const state = { nonce: '123', env: 'development', origin: undefined };
+
+      const encoded = encodeState(state);
+      expect(encoded).toBe(
+        Buffer.from('nonce=123&env=development').toString('hex'),
+      );
+
+      expect(readState(encoded)).toEqual(state);
+    });
+  });
+
   describe('verifyNonce', () => {
     it('should throw error if cookie nonce missing', () => {
       const state = { nonce: 'NONCE', env: 'development' };
diff --git a/plugins/auth-backend/src/lib/oauth/helpers.ts b/plugins/auth-backend/src/lib/oauth/helpers.ts
index ead9acae27..17e3769d21 100644
--- a/plugins/auth-backend/src/lib/oauth/helpers.ts
+++ b/plugins/auth-backend/src/lib/oauth/helpers.ts
@@ -16,6 +16,7 @@
 
 import express from 'express';
 import { OAuthState } from './types';
+import pickBy from 'lodash/pickBy';
 
 export const readState = (stateString: string): OAuthState => {
   const state = Object.fromEntries(
@@ -34,7 +35,9 @@ export const readState = (stateString: string): OAuthState => {
 };
 
 export const encodeState = (state: OAuthState): string => {
-  const stateString = new URLSearchParams(state).toString();
+  const stateString = new URLSearchParams(
+    pickBy(state, value => value !== undefined),
+  ).toString();
 
   return Buffer.from(stateString, 'utf-8').toString('hex');
 };