diff --git a/changelog.d/13722.feature b/changelog.d/13722.feature
new file mode 100644
index 0000000000..588d143c0f
--- /dev/null
+++ b/changelog.d/13722.feature
@@ -0,0 +1 @@
+Experimental implementation of MSC3882 to allow an existing device/session to generate a login token for use on a new device/session.
diff --git a/synapse/config/experimental.py b/synapse/config/experimental.py
index f4541a8db0..bf27f6c101 100644
--- a/synapse/config/experimental.py
+++ b/synapse/config/experimental.py
@@ -96,3 +96,10 @@ class ExperimentalConfig(Config):
 
         # MSC3881: Remotely toggle push notifications for another client
         self.msc3881_enabled: bool = experimental.get("msc3881_enabled", False)
+
+        # MSC3882: Allow an existing session to sign in a new session
+        self.msc3882_enabled: bool = experimental.get("msc3882_enabled", False)
+        self.msc3882_ui_auth: bool = experimental.get("msc3882_ui_auth", True)
+        self.msc3882_token_timeout = self.parse_duration(
+            experimental.get("msc3882_token_timeout", "5m")
+        )
diff --git a/synapse/rest/__init__.py b/synapse/rest/__init__.py
index b712215112..9a2ab99ede 100644
--- a/synapse/rest/__init__.py
+++ b/synapse/rest/__init__.py
@@ -30,6 +30,7 @@ from synapse.rest.client import (
     keys,
     knock,
     login as v1_login,
+    login_token_request,
     logout,
     mutual_rooms,
     notifications,
@@ -130,3 +131,4 @@ class ClientRestResource(JsonResource):
 
         # unstable
         mutual_rooms.register_servlets(hs, client_resource)
+        login_token_request.register_servlets(hs, client_resource)
diff --git a/synapse/rest/client/login_token_request.py b/synapse/rest/client/login_token_request.py
new file mode 100644
index 0000000000..ca5c54bf17
--- /dev/null
+++ b/synapse/rest/client/login_token_request.py
@@ -0,0 +1,94 @@
+# Copyright 2022 The Matrix.org Foundation C.I.C.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+import logging
+from typing import TYPE_CHECKING, Tuple
+
+from synapse.http.server import HttpServer
+from synapse.http.servlet import RestServlet, parse_json_object_from_request
+from synapse.http.site import SynapseRequest
+from synapse.rest.client._base import client_patterns, interactive_auth_handler
+from synapse.types import JsonDict
+
+if TYPE_CHECKING:
+    from synapse.server import HomeServer
+
+logger = logging.getLogger(__name__)
+
+
+class LoginTokenRequestServlet(RestServlet):
+    """
+    Get a token that can be used with `m.login.token` to log in a second device.
+
+    Request:
+
+    POST /login/token HTTP/1.1
+    Content-Type: application/json
+
+    {}
+
+    Response:
+
+    HTTP/1.1 200 OK
+    {
+        "login_token": "ABDEFGH",
+        "expires_in": 3600,
+    }
+    """
+
+    PATTERNS = client_patterns("/login/token$")
+
+    def __init__(self, hs: "HomeServer"):
+        super().__init__()
+        self.auth = hs.get_auth()
+        self.store = hs.get_datastores().main
+        self.clock = hs.get_clock()
+        self.server_name = hs.config.server.server_name
+        self.macaroon_gen = hs.get_macaroon_generator()
+        self.auth_handler = hs.get_auth_handler()
+        self.token_timeout = hs.config.experimental.msc3882_token_timeout
+        self.ui_auth = hs.config.experimental.msc3882_ui_auth
+
+    @interactive_auth_handler
+    async def on_POST(self, request: SynapseRequest) -> Tuple[int, JsonDict]:
+        requester = await self.auth.get_user_by_req(request)
+        body = parse_json_object_from_request(request)
+
+        if self.ui_auth:
+            await self.auth_handler.validate_user_via_ui_auth(
+                requester,
+                request,
+                body,
+                "issue a new access token for your account",
+                can_skip_ui_auth=False,  # Don't allow skipping of UI auth
+            )
+
+        login_token = self.macaroon_gen.generate_short_term_login_token(
+            user_id=requester.user.to_string(),
+            auth_provider_id="org.matrix.msc3882.login_token_request",
+            duration_in_ms=self.token_timeout,
+        )
+
+        return (
+            200,
+            {
+                "login_token": login_token,
+                "expires_in": self.token_timeout // 1000,
+            },
+        )
+
+
+def register_servlets(hs: "HomeServer", http_server: HttpServer) -> None:
+    if hs.config.experimental.msc3882_enabled:
+        LoginTokenRequestServlet(hs).register(http_server)
diff --git a/synapse/rest/client/versions.py b/synapse/rest/client/versions.py
index c516cda95d..c3488f4330 100644
--- a/synapse/rest/client/versions.py
+++ b/synapse/rest/client/versions.py
@@ -105,6 +105,8 @@ class VersionsRestServlet(RestServlet):
                     "org.matrix.msc3440.stable": True,  # TODO: remove when "v1.3" is added above
                     # Allows moderators to fetch redacted event content as described in MSC2815
                     "fi.mau.msc2815": self.config.experimental.msc2815_enabled,
+                    # Adds support for login token requests as per MSC3882
+                    "org.matrix.msc3882": self.config.experimental.msc3882_enabled,
                 },
             },
         )
diff --git a/tests/rest/client/test_login_token_request.py b/tests/rest/client/test_login_token_request.py
new file mode 100644
index 0000000000..d5bb16c98d
--- /dev/null
+++ b/tests/rest/client/test_login_token_request.py
@@ -0,0 +1,132 @@
+# Copyright 2022 The Matrix.org Foundation C.I.C.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+from twisted.test.proto_helpers import MemoryReactor
+
+from synapse.rest import admin
+from synapse.rest.client import login, login_token_request
+from synapse.server import HomeServer
+from synapse.util import Clock
+
+from tests import unittest
+from tests.unittest import override_config
+
+
+class LoginTokenRequestServletTestCase(unittest.HomeserverTestCase):
+
+    servlets = [
+        login.register_servlets,
+        admin.register_servlets,
+        login_token_request.register_servlets,
+    ]
+
+    def make_homeserver(self, reactor: MemoryReactor, clock: Clock) -> HomeServer:
+        self.hs = self.setup_test_homeserver()
+        self.hs.config.registration.enable_registration = True
+        self.hs.config.registration.registrations_require_3pid = []
+        self.hs.config.registration.auto_join_rooms = []
+        self.hs.config.captcha.enable_registration_captcha = False
+
+        return self.hs
+
+    def prepare(self, reactor: MemoryReactor, clock: Clock, hs: HomeServer) -> None:
+        self.user = "user123"
+        self.password = "password"
+
+    def test_disabled(self) -> None:
+        channel = self.make_request("POST", "/login/token", {}, access_token=None)
+        self.assertEqual(channel.code, 400)
+
+        self.register_user(self.user, self.password)
+        token = self.login(self.user, self.password)
+
+        channel = self.make_request("POST", "/login/token", {}, access_token=token)
+        self.assertEqual(channel.code, 400)
+
+    @override_config({"experimental_features": {"msc3882_enabled": True}})
+    def test_require_auth(self) -> None:
+        channel = self.make_request("POST", "/login/token", {}, access_token=None)
+        self.assertEqual(channel.code, 401)
+
+    @override_config({"experimental_features": {"msc3882_enabled": True}})
+    def test_uia_on(self) -> None:
+        user_id = self.register_user(self.user, self.password)
+        token = self.login(self.user, self.password)
+
+        channel = self.make_request("POST", "/login/token", {}, access_token=token)
+        self.assertEqual(channel.code, 401)
+        self.assertIn({"stages": ["m.login.password"]}, channel.json_body["flows"])
+
+        session = channel.json_body["session"]
+
+        uia = {
+            "auth": {
+                "type": "m.login.password",
+                "identifier": {"type": "m.id.user", "user": self.user},
+                "password": self.password,
+                "session": session,
+            },
+        }
+
+        channel = self.make_request("POST", "/login/token", uia, access_token=token)
+        self.assertEqual(channel.code, 200)
+        self.assertEqual(channel.json_body["expires_in"], 300)
+
+        login_token = channel.json_body["login_token"]
+
+        channel = self.make_request(
+            "POST",
+            "/login",
+            content={"type": "m.login.token", "token": login_token},
+        )
+        self.assertEqual(channel.code, 200, channel.result)
+        self.assertEqual(channel.json_body["user_id"], user_id)
+
+    @override_config(
+        {"experimental_features": {"msc3882_enabled": True, "msc3882_ui_auth": False}}
+    )
+    def test_uia_off(self) -> None:
+        user_id = self.register_user(self.user, self.password)
+        token = self.login(self.user, self.password)
+
+        channel = self.make_request("POST", "/login/token", {}, access_token=token)
+        self.assertEqual(channel.code, 200)
+        self.assertEqual(channel.json_body["expires_in"], 300)
+
+        login_token = channel.json_body["login_token"]
+
+        channel = self.make_request(
+            "POST",
+            "/login",
+            content={"type": "m.login.token", "token": login_token},
+        )
+        self.assertEqual(channel.code, 200, channel.result)
+        self.assertEqual(channel.json_body["user_id"], user_id)
+
+    @override_config(
+        {
+            "experimental_features": {
+                "msc3882_enabled": True,
+                "msc3882_ui_auth": False,
+                "msc3882_token_timeout": "15s",
+            }
+        }
+    )
+    def test_expires_in(self) -> None:
+        self.register_user(self.user, self.password)
+        token = self.login(self.user, self.password)
+
+        channel = self.make_request("POST", "/login/token", {}, access_token=token)
+        self.assertEqual(channel.code, 200)
+        self.assertEqual(channel.json_body["expires_in"], 15)