diff --git a/packages/smithy-http/.changes/next-release/smithy-http-bugfix-20251017171341.json b/packages/smithy-http/.changes/next-release/smithy-http-bugfix-20251017171341.json
new file mode 100644
index 00000000..d8880d55
--- /dev/null
+++ b/packages/smithy-http/.changes/next-release/smithy-http-bugfix-20251017171341.json
@@ -0,0 +1,4 @@
+{
+  "type": "bugfix",
+  "description": "add port to CRT HTTP client's host header"
+}
\ No newline at end of file
diff --git a/packages/smithy-http/src/smithy_http/aio/crt.py b/packages/smithy-http/src/smithy_http/aio/crt.py
index 4dd3232b..a450ef9c 100644
--- a/packages/smithy-http/src/smithy_http/aio/crt.py
+++ b/packages/smithy-http/src/smithy_http/aio/crt.py
@@ -273,7 +273,7 @@ def _marshal_request(
         headers_list: list[tuple[str, str]] = []
         if "host" not in request.fields:
             request.fields.set_field(
-                Field(name="host", values=[request.destination.host])
+                Field(name="host", values=[request.destination.netloc])
             )
 
         if "accept" not in request.fields:
diff --git a/packages/smithy-http/tests/unit/aio/test_crt.py b/packages/smithy-http/tests/unit/aio/test_crt.py
index 5b9e851e..886bb5f2 100644
--- a/packages/smithy-http/tests/unit/aio/test_crt.py
+++ b/packages/smithy-http/tests/unit/aio/test_crt.py
@@ -45,6 +45,27 @@ def test_client_marshal_request() -> None:
     assert crt_request.path == "/path?key1=value1&key2=value2"
 
 
+@pytest.mark.parametrize(
+    "host,expected",
+    [
+        ("example.com", "example.com:8443"),
+        ("2001:db8::1", "[2001:db8::1]:8443"),
+    ],
+)
+async def test_port_included_in_host_header(host: str, expected: str) -> None:
+    client = AWSCRTHTTPClient()
+    request = HTTPRequest(
+        method="GET",
+        destination=URI(
+            host=host, path="/path", query="key1=value1&key2=value2", port=8443
+        ),
+        body=BytesIO(),
+        fields=Fields(),
+    )
+    crt_request = client._marshal_request(request)  # type: ignore
+    assert crt_request.headers.get("host") == expected  # type: ignore
+
+
 async def test_body_generator_bytes() -> None:
     """Test body generator with bytes input."""
     client = AWSCRTHTTPClient()