diff --git a/synapse/config/experimental.py b/synapse/config/experimental.py
index 5aa3b3a75c..d7454bbf33 100644
--- a/synapse/config/experimental.py
+++ b/synapse/config/experimental.py
@@ -248,6 +248,19 @@ class ExperimentalConfig(Config):
                 ("experimental", "msc4388_enabled"),
             )
 
+        # How long a rendezvous session lasts before it expires. MSC4388
+        # recommends at least 2 minutes for usability, to allow the user time to
+        # complete the sign in including any extra steps such as MFA, and at
+        # most 5 minutes for security.
+        self.msc4388_session_ttl_ms = self.parse_duration(
+            experimental.get("msc4388_session_ttl", "2m")
+        )
+        if not 2 * 60 * 1000 <= self.msc4388_session_ttl_ms <= 5 * 60 * 1000:
+            raise ConfigError(
+                "msc4388_session_ttl must be between 2 and 5 minutes",
+                ("experimental", "msc4388_session_ttl"),
+            )
+
         # MSC4388 requests are ratelimited by client IP address, as most are
         # unauthenticated: only discovery and creating a session may carry an
         # access token, and even then it is not used for ratelimiting.
@@ -255,8 +268,8 @@ class ExperimentalConfig(Config):
         # Creating a session is limited more strictly than other requests, as
         # there is a cap on the number of concurrent sessions, and creating new
         # sessions beyond it evicts the oldest ones. A client using these
-        # defaults can hold at most around 20 live sessions at once, as each
-        # session expires after 2 minutes.
+        # defaults can hold at most around 20 live sessions at once with the
+        # default session TTL of 2 minutes, or 40 with the maximum of 5 minutes.
         self.msc4388_rc_create = RatelimitSettings.parse(
             experimental,
             "msc4388_rc_create",
diff --git a/synapse/server.py b/synapse/server.py
index 266440534c..a6a4ffac67 100644
--- a/synapse/server.py
+++ b/synapse/server.py
@@ -1209,7 +1209,9 @@ class HomeServer(metaclass=abc.ABCMeta):
 
     @cache_in_self
     def get_msc4388_rendezvous_handler(self) -> MSC4388RendezvousHandler:
-        return MSC4388RendezvousHandler(self)
+        return MSC4388RendezvousHandler(
+            self, ttl=self.config.experimental.msc4388_session_ttl_ms
+        )
 
     @cache_in_self
     def get_outbound_redis_connection(self) -> "ConnectionHandler":
diff --git a/tests/config/test_experimental.py b/tests/config/test_experimental.py
index 0fede98ee9..a18c03f2be 100644
--- a/tests/config/test_experimental.py
+++ b/tests/config/test_experimental.py
@@ -15,6 +15,7 @@
 import yaml
 from parameterized import parameterized
 
+from synapse.config import ConfigError
 from synapse.config._base import RootConfig
 from synapse.config.experimental import ExperimentalConfig
 from synapse.config.homeserver import HomeServerConfig
@@ -56,9 +57,39 @@ class ExperimentalConfigTestCase(unittest.TestCase):
 
         _read_config(config_values)
 
+    @parameterized.expand(
+        [
+            ["default", None, 2 * 60 * 1000],
+            ["minimum", "2m", 2 * 60 * 1000],
+            ["maximum", "5m", 5 * 60 * 1000],
+            ["milliseconds", 200000, 200000],
+        ]
+    )
+    def test_msc4388_session_ttl(
+        self, test_description: str, ttl: str | int | None, expected_ms: int
+    ) -> None:
+        """
+        Test that the MSC4388 session TTL is parsed as a duration.
+        """
+        experimental_features = {} if ttl is None else {"msc4388_session_ttl": ttl}
+        config = _read_config({"experimental_features": experimental_features})
+        self.assertEqual(config.msc4388_session_ttl_ms, expected_ms)
 
-def _read_config(config_values: JsonDict) -> None:
-    ExperimentalConfig(RootConfig()).read_config(
+    @parameterized.expand([["too_short", "119s"], ["too_long", "301s"]])
+    def test_msc4388_session_ttl_out_of_range(
+        self, test_description: str, ttl: str
+    ) -> None:
+        """
+        Test that an MSC4388 session TTL outside of the range recommended by the
+        MSC is rejected.
+        """
+        with self.assertRaises(ConfigError):
+            _read_config({"experimental_features": {"msc4388_session_ttl": ttl}})
+
+
+def _read_config(config_values: JsonDict) -> ExperimentalConfig:
+    config = ExperimentalConfig(RootConfig())
+    config.read_config(
         yaml.safe_load(
             HomeServerConfig().generate_config(
                 config_dir_path="CONFDIR",
@@ -69,3 +100,4 @@ def _read_config(config_values: JsonDict) -> None:
         | config_values,
         allow_secrets_in_config=False,
     )
+    return config
diff --git a/tests/rest/client/test_msc4388_rendezvous.py b/tests/rest/client/test_msc4388_rendezvous.py
index ec326202cd..eb09be8e5f 100644
--- a/tests/rest/client/test_msc4388_rendezvous.py
+++ b/tests/rest/client/test_msc4388_rendezvous.py
@@ -622,6 +622,34 @@ class RendezvousServletTestCase(unittest.HomeserverTestCase):
         )
         self.assertEqual(channel.code, 404)
 
+    @override_config({"experimental_features": {"msc4388_session_ttl": "5m"}})
+    def test_configured_ttl(self) -> None:
+        """
+        Test that sessions expire after the configured TTL.
+        """
+        channel = self.make_request(
+            "POST",
+            rz_endpoint,
+            {"data": "foo=bar"},
+            access_token=None,
+        )
+        self.assertEqual(channel.code, 200)
+        self.assertEqual(channel.json_body["expires_in_ms"], 5 * 60 * 1000)
+        session_endpoint = rz_endpoint + "/" + channel.json_body["id"]
+
+        # The session is still there after the default TTL of 2 minutes...
+        self.reactor.advance(120)
+        channel = self.make_request("GET", session_endpoint, access_token=None)
+        self.assertEqual(channel.code, 200)
+        # (allowing for the time taken by the requests)
+        self.assertLessEqual(channel.json_body["expires_in_ms"], 3 * 60 * 1000)
+        self.assertGreater(channel.json_body["expires_in_ms"], 3 * 60 * 1000 - 1000)
+
+        # ...but is gone after the configured TTL of 5 minutes
+        self.reactor.advance(180)
+        channel = self.make_request("GET", session_endpoint, access_token=None)
+        self.assertEqual(channel.code, 404)
+
     def test_delete_expired(self) -> None:
         """
         Test that deleting a session which has expired, but has not yet been