diff --git a/src/lib/isURL.js b/src/lib/isURL.js
index 8ae971ed6..7c20cbffe 100644
--- a/src/lib/isURL.js
+++ b/src/lib/isURL.js
@@ -113,8 +113,14 @@ export default function isURL(url, options) {
     // 2. There's an `@` symbol before any `/`
     // 3. The part before `@` contains only valid auth characters (alphanumeric, -, _, ., %, :)
     const starts_with_slashes = after_colon.slice(0, 2) === '//';
+    const is_port = /^[0-9]+(?:\/|$)/.test(after_colon);
 
-    if (!starts_with_slashes) {
+    if (!starts_with_slashes && is_port) {
+      // This is a hostname with a port, not a protocol (e.g., localhost:3000)
+      if (options.require_protocol) {
+        return false;
+      }
+    } else if (!starts_with_slashes) {
       const first_slash_position = after_colon.indexOf('/');
       const before_slash = first_slash_position === -1
         ? after_colon
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',