diff --git a/integrationExamples/gpt/userId_example.html b/integrationExamples/gpt/userId_example.html
index e6f9255326a..fa9b1e3b5fd 100644
--- a/integrationExamples/gpt/userId_example.html
+++ b/integrationExamples/gpt/userId_example.html
@@ -109,6 +109,7 @@
         <script src="../../build/dev/prebid.js" async></script>
 
         <script>
+            function getHashedEmail() {}
             var googletag = googletag || {};
             googletag.cmd = googletag.cmd || [];
             googletag.cmd.push(function() {
@@ -157,7 +158,7 @@
                                         }
                                     }]
                                 }],
-                                eidsFunction: getHashedEmail  // any user defined function that exists in the page
+                                eidsFunction: getHashedEmail // any user defined function that exists in the page
                             }
                         },{
                             name: "unifiedId",
diff --git a/modules/userId/index.js b/modules/userId/index.js
index f063fbc973b..9294311de69 100644
--- a/modules/userId/index.js
+++ b/modules/userId/index.js
@@ -48,6 +48,20 @@
  * @type {string}
  */
 
+/**
+ * @property
+ * @summary use a predefined domain override for cookies or provide your own
+ * @name Submodule#domainOverride
+ * @type {(undefined|function)}
+ */
+
+/**
+ * @function
+ * @summary Returns the root domain
+ * @name Submodule#findRootDomain
+ * @returns {string}
+ */
+
 /**
  * @typedef {Object} SubmoduleConfig
  * @property {string} name - the User ID submodule name (used to link submodule with config)
@@ -315,6 +329,60 @@ function hasGDPRConsent(consentData) {
   return true;
 }
 
+/**
+   * Find the root domain
+   * @param {string|undefined} fullDomain
+   * @return {string}
+   */
+export function findRootDomain(fullDomain = window.location.hostname) {
+  if (!coreStorage.cookiesAreEnabled()) {
+    return fullDomain;
+  }
+
+  const domainParts = fullDomain.split('.');
+  if (domainParts.length == 2) {
+    return fullDomain;
+  }
+  let rootDomain;
+  let continueSearching;
+  let startIndex = -2;
+  const TEST_COOKIE_NAME = `_rdc${Date.now()}`;
+  const TEST_COOKIE_VALUE = 'writeable';
+  do {
+    rootDomain = domainParts.slice(startIndex).join('.');
+    let expirationDate = new Date(utils.timestamp() + 10 * 1000).toUTCString();
+
+    // Write a test cookie
+    coreStorage.setCookie(
+      TEST_COOKIE_NAME,
+      TEST_COOKIE_VALUE,
+      expirationDate,
+      'Lax',
+      rootDomain,
+      undefined
+    );
+
+    // See if the write was successful
+    const value = coreStorage.getCookie(TEST_COOKIE_NAME, undefined);
+    if (value === TEST_COOKIE_VALUE) {
+      continueSearching = false;
+      // Delete our test cookie
+      coreStorage.setCookie(
+        TEST_COOKIE_NAME,
+        '',
+        'Thu, 01 Jan 1970 00:00:01 GMT',
+        undefined,
+        rootDomain,
+        undefined
+      );
+    } else {
+      startIndex += -1;
+      continueSearching = Math.abs(startIndex) <= domainParts.length;
+    }
+  } while (continueSearching);
+  return rootDomain;
+}
+
 /**
  * @param {SubmoduleContainer[]} submodules
  * @param {function} cb - callback for after processing is done.
@@ -657,6 +725,7 @@ function updateSubmodules() {
   // find submodule and the matching configuration, if found create and append a SubmoduleContainer
   submodules = addedSubmodules.map(i => {
     const submoduleConfig = find(configs, j => j.name === i.name);
+    i.findRootDomain = findRootDomain;
     return submoduleConfig ? {
       submodule: i,
       config: submoduleConfig,
diff --git a/test/spec/modules/userId_spec.js b/test/spec/modules/userId_spec.js
index 981ebb5f50e..9eace12cd2b 100644
--- a/test/spec/modules/userId_spec.js
+++ b/test/spec/modules/userId_spec.js
@@ -8,7 +8,8 @@ import {
   setStoredValue,
   setSubmoduleRegistry,
   syncDelay,
-  PBJS_USER_ID_OPTOUT_NAME
+  PBJS_USER_ID_OPTOUT_NAME,
+  findRootDomain,
 } from 'modules/userId/index.js';
 import {createEidsArray} from 'modules/userId/eids.js';
 import {config} from 'src/config.js';
@@ -2145,6 +2146,8 @@ describe('User ID', function () {
     let coreStorageSpy;
     beforeEach(function () {
       coreStorageSpy = sinon.spy(coreStorage, 'setCookie');
+      setSubmoduleRegistry([pubCommonIdSubmodule]);
+      init(config);
     });
     afterEach(function () {
       coreStorageSpy.restore();
@@ -2360,5 +2363,46 @@ describe('User ID', function () {
         sinon.assert.calledOnce(mockExtendId);
       });
     });
+
+    describe('findRootDomain', function () {
+      let sandbox;
+
+      beforeEach(function () {
+        setSubmoduleRegistry([pubCommonIdSubmodule]);
+        init(config);
+        config.setConfig({
+          userSync: {
+            syncDelay: 0,
+            userIds: [
+              {
+                name: 'pubCommonId',
+                value: { pubcid: '11111' },
+              },
+            ],
+          },
+        });
+        sandbox = sinon.createSandbox();
+        sandbox
+          .stub(coreStorage, 'getCookie')
+          .onFirstCall()
+          .returns(null) // .co.uk
+          .onSecondCall()
+          .returns('writeable'); // realdomain.co.uk;
+      });
+
+      afterEach(function () {
+        sandbox.restore();
+      });
+
+      it('should just find the root domain', function () {
+        var domain = findRootDomain('sub.realdomain.co.uk');
+        expect(domain).to.be.eq('realdomain.co.uk');
+      });
+
+      it('should find the full domain when no subdomain is present', function () {
+        var domain = findRootDomain('realdomain.co.uk');
+        expect(domain).to.be.eq('realdomain.co.uk');
+      });
+    });
   });
 });