diff --git a/modules/undertoneBidAdapter.js b/modules/undertoneBidAdapter.js
index 1f32fe0347a..d7e063b85f5 100644
--- a/modules/undertoneBidAdapter.js
+++ b/modules/undertoneBidAdapter.js
@@ -21,11 +21,15 @@ export const spec = {
       'x-ut-hb-params': []
     };
     const location = utils.getTopWindowLocation();
-    let domain = /[-\w]+\.(?:[-\w]+\.xn--[-\w]+|[-\w]{3,}|[-\w]+\.[-\w]{2})$/i.exec(location.host);
-    if (domain == null || domain.length == 0) {
-      domain = null;
-    } else {
-      domain = domain[0];
+    let domains = /[-\w]+\.([-\w]+|[-\w]{3,}|[-\w]{1,3}\.[-\w]{2})$/i.exec(location.host);
+    let domain = null;
+    if (domains != null && domains.length > 0) {
+      domain = domains[0];
+      for (let i = 1; i < domains.length; i++) {
+        if (domains[i].length > domain.length) {
+          domain = domains[i];
+        }
+      }
     }
 
     const pubid = validBidRequests[0].params.publisherId;