diff --git a/src/flask/testing.py b/src/flask/testing.py
index 68b1ab48c6..809069f59a 100644
--- a/src/flask/testing.py
+++ b/src/flask/testing.py
@@ -109,8 +109,8 @@ def _get_werkzeug_version() -> str:
 class FlaskClient(Client):
     """Works like a regular Werkzeug test client, with additional behavior for
     Flask. Can defer the cleanup of the request context until the end of a
-    ``with`` block. For general information about how to use this class refer to
-    :class:`werkzeug.test.Client`.
+    ``with`` block. For general information about how to use this class refer
+    to :class:`werkzeug.test.Client`.
 
     .. versionchanged:: 0.12
        `app.test_client()` includes preset default environment, which can be
@@ -177,7 +177,7 @@ def session_transaction(
             app.session_interface.save_session(app, sess, resp)
 
         self._update_cookies_from_response(
-            ctx.request.host.partition(":")[0],
+            urlsplit(ctx.request.host_url).hostname or "localhost",
             ctx.request.path,
             resp.headers.getlist("Set-Cookie"),
         )
diff --git a/tests/test_testing.py b/tests/test_testing.py
index c172f14d68..4c014f2e7c 100644
--- a/tests/test_testing.py
+++ b/tests/test_testing.py
@@ -171,6 +171,20 @@ def index():
             assert sess["foo"] == [42]
 
 
+def test_session_transaction_ipv6(app):
+    base_url = "http://[::1]:8000/"
+    client = app.test_client()
+
+    @app.get("/")
+    def index():
+        return str(flask.session.get("value"))
+
+    with client.session_transaction(base_url=base_url) as sess:
+        sess["value"] = 42
+
+    assert client.get("/", base_url=base_url).text == "42"
+
+
 def test_session_transactions_no_null_sessions():
     app = flask.Flask(__name__)