diff --git a/mongoengine/fields.py b/mongoengine/fields.py
index d812a762..eec31829 100644
--- a/mongoengine/fields.py
+++ b/mongoengine/fields.py
@@ -121,7 +121,7 @@ class URLField(StringField):
 
     _URL_REGEX = re.compile(
         r'^(?:[a-z0-9\.\-]*)://'  # scheme is validated separately
-        r'(?:(?:[A-Z0-9](?:[A-Z0-9-]{0,61}[A-Z0-9])?\.)+(?:[A-Z]{2,6}\.?|[A-Z0-9-]{2,}(?<!-)\.?)|'  # domain...
+        r'(?:(?:[A-Z0-9](?:[A-Z0-9-_]{0,61}[A-Z0-9])?\.)+(?:[A-Z]{2,6}\.?|[A-Z0-9-]{2,}(?<!-)\.?)|'  # domain...
         r'localhost|'  # localhost...
         r'\d{1,3}\.\d{1,3}\.\d{1,3}\.\d{1,3}|'  # ...or ipv4
         r'\[?[A-F0-9]*:[A-F0-9:]+\]?)'  # ...or ipv6
diff --git a/tests/fields/fields.py b/tests/fields/fields.py
index 678786fa..482f21d5 100644
--- a/tests/fields/fields.py
+++ b/tests/fields/fields.py
@@ -361,6 +361,16 @@ class FieldTest(unittest.TestCase):
         scheme_link.url = 'ws://google.com'
         scheme_link.validate()
 
+    def test_url_allowed_domains(self):
+        """Allow underscore in domain names.
+        """
+        class Link(Document):
+            url = URLField()
+
+        link = Link()
+        link.url = 'https://san_leandro-ca.geebo.com'
+        link.validate()
+
     def test_int_validation(self):
         """Ensure that invalid values cannot be assigned to int fields.
         """