diff --git a/modules/33acrossIdSystem.js b/modules/33acrossIdSystem.js
index 33086562111..e0f7435a1ec 100644
--- a/modules/33acrossIdSystem.js
+++ b/modules/33acrossIdSystem.js
@@ -80,7 +80,7 @@ function calculateQueryStringParams(pid, gdprConsentData, storageConfig) {
 
   const fp = getStoredValue(STORAGE_FPID_KEY, storageConfig);
   if (fp) {
-    params.fp = fp;
+    params.fp = encodeURIComponent(fp);
   }
 
   return params;
diff --git a/test/spec/modules/33acrossIdSystem_spec.js b/test/spec/modules/33acrossIdSystem_spec.js
index cbc5b277e30..364f7d2845c 100644
--- a/test/spec/modules/33acrossIdSystem_spec.js
+++ b/test/spec/modules/33acrossIdSystem_spec.js
@@ -493,7 +493,7 @@ describe('33acrossIdSystem', () => {
     });
 
     context('when a first-party ID is present in local storage', () => {
-      it('should call endpoint with the first-party ID included', () => {
+      it('should call endpoint with the encoded first-party ID included', () => {
         const completeCallback = () => {};
         const { callback } = thirthyThreeAcrossIdSubmodule.getId({
           params: {
@@ -506,13 +506,13 @@ describe('33acrossIdSystem', () => {
 
         sinon.stub(storage, 'getDataFromLocalStorage')
           .withArgs('33acrossIdFp')
-          .returns('33acrossIdFpValue');
+          .returns('33acrossIdFpValue+');
 
         callback(completeCallback);
 
         const [request] = server.requests;
 
-        expect(request.url).to.contain('fp=33acrossIdFpValue');
+        expect(request.url).to.contain('fp=33acrossIdFpValue%2B');
 
         storage.getDataFromLocalStorage.restore();
       });