diff --git a/starlette/datastructures.py b/starlette/datastructures.py
index ebef2ebdfa..fe9ff6e45e 100644
--- a/starlette/datastructures.py
+++ b/starlette/datastructures.py
@@ -114,11 +114,18 @@ def replace(self, **kwargs: typing.Any) -> "URL":
             or "hostname" in kwargs
             or "port" in kwargs
         ):
-            hostname = kwargs.pop("hostname", self.hostname)
+            hostname = kwargs.pop("hostname", None)
             port = kwargs.pop("port", self.port)
             username = kwargs.pop("username", self.username)
             password = kwargs.pop("password", self.password)
 
+            if not hostname:
+                netloc = self.netloc
+                _, _, hostname = netloc.rpartition("@")
+
+                if hostname[-1] != "]":
+                    hostname = hostname.rsplit(":", 1)[0]
+
             netloc = hostname
             if port is not None:
                 netloc += f":{port}"
diff --git a/tests/test_datastructures.py b/tests/test_datastructures.py
index b87b26e22f..5c4cb34cf0 100644
--- a/tests/test_datastructures.py
+++ b/tests/test_datastructures.py
@@ -38,6 +38,24 @@ def test_url():
     assert new == "https://example.com:123/path/to/somewhere?abc=123#anchor"
     assert new.hostname == "example.com"
 
+    ipv6_url = URL("https://[fe::2]:12345")
+    new = ipv6_url.replace(port=8080)
+    assert new == "https://[fe::2]:8080"
+
+    new = ipv6_url.replace(username="username", password="password")
+    assert new == "https://username:password@[fe::2]:12345"
+    assert new.netloc == "username:password@[fe::2]:12345"
+
+    ipv6_url = URL("https://[fe::2]")
+    new = ipv6_url.replace(port=123)
+    assert new == "https://[fe::2]:123"
+
+    url = URL("http://u:p@host/")
+    assert url.replace(hostname="bar") == URL("http://u:p@bar/")
+
+    url = URL("http://u:p@host:80")
+    assert url.replace(port=88) == URL("http://u:p@host:88")
+
 
 def test_url_query_params():
     u = URL("https://example.org/path/?page=3")