diff --git a/src/lib/isURL.js b/src/lib/isURL.js
index 8ae971ed6..560f9b041 100644
--- a/src/lib/isURL.js
+++ b/src/lib/isURL.js
@@ -120,6 +120,7 @@ export default function isURL(url, options) {
         ? after_colon
         : after_colon.substring(0, first_slash_position);
       const at_position = before_slash.indexOf('@');
+      const looks_like_port = /^[0-9]+$/.test(before_slash);
 
       if (at_position !== -1) {
         const before_at = before_slash.substring(0, at_position);
@@ -141,8 +142,8 @@ export default function isURL(url, options) {
             return false;
           }
         }
-      } else {
-        // No @ symbol, this is definitely a protocol
+      } else if (options.require_protocol || !looks_like_port) {
+        // No @ symbol and not an allowed host:port URL, so treat it as a protocol.
         url = cleanUpProtocol(potential_protocol);
 
         if (url === false) {
diff --git a/test/validators.test.js b/test/validators.test.js
index 5196c6fe9..9db1f1745 100644
--- a/test/validators.test.js
+++ b/test/validators.test.js
@@ -499,6 +499,7 @@ describe('Validators', () => {
         'localhost',
         'localhost:3000',
         'service-name:8080',
+        'my-3272-service:9000',
         'https://localhost',
         'http://localhost:3000',
         'http://service-name:8080',