diff --git a/changelog.d/12391.feature b/changelog.d/12391.feature
new file mode 100644
index 000000000000..9a064ec8be73
--- /dev/null
+++ b/changelog.d/12391.feature
@@ -0,0 +1 @@
+Add a module API for reading and writing global account data.
diff --git a/synapse/module_api/__init__.py b/synapse/module_api/__init__.py
index 9a61593ff511..8f9e62927465 100644
--- a/synapse/module_api/__init__.py
+++ b/synapse/module_api/__init__.py
@@ -119,6 +119,7 @@
 from synapse.util import Clock
 from synapse.util.async_helpers import maybe_awaitable
 from synapse.util.caches.descriptors import cached
+from synapse.util.frozenutils import freeze
 
 if TYPE_CHECKING:
     from synapse.app.generic_worker import GenericWorkerSlavedStore
@@ -211,6 +212,7 @@ def __init__(self, hs: "HomeServer", auth_handler: AuthHandler) -> None:
         # We expose these as properties below in order to attach a helpful docstring.
         self._http_client: SimpleHttpClient = hs.get_simple_http_client()
         self._public_room_list_manager = PublicRoomListManager(hs)
+        self._account_data_manager = AccountDataManager(hs)
 
         self._spam_checker = hs.get_spam_checker()
         self._account_validity_handler = hs.get_account_validity_handler()
@@ -431,6 +433,14 @@ def public_room_list_manager(self) -> "PublicRoomListManager":
         """
         return self._public_room_list_manager
 
+    @property
+    def account_data_manager(self) -> "AccountDataManager":
+        """Allows reading and modifying users' account data.
+
+        Added in Synapse v1.57.0.
+        """
+        return self._account_data_manager
+
     @property
     def public_baseurl(self) -> str:
         """The configured public base URL for this homeserver.
@@ -1386,3 +1396,69 @@ async def remove_room_from_public_room_list(self, room_id: str) -> None:
             room_id: The ID of the room.
         """
         await self._store.set_room_is_public(room_id, False)
+
+
+class AccountDataManager:
+    """
+    Allows modules to manage account data.
+    """
+
+    def __init__(self, hs: "HomeServer") -> None:
+        self._hs = hs
+        self._store = hs.get_datastores().main
+        self._handler = hs.get_account_data_handler()
+
+    def _validate_user_id(self, user_id: str) -> None:
+        """
+        Validates a user ID is valid and local.
+        Private method to be used in other account data methods.
+        """
+        user = UserID.from_string(user_id)
+        if not self._hs.is_mine(user):
+            raise ValueError(
+                f"{user_id} is not local to this homeserver; can't access account data for remote users."
+            )
+
+    async def get_global(self, user_id: str, data_type: str) -> Optional[JsonDict]:
+        """
+        Gets some global account data, of a specified type, for the specified user.
+
+        The provided user ID must be a valid user ID of a local user.
+
+        Added in Synapse v1.57.0.
+        """
+        self._validate_user_id(user_id)
+
+        data = await self._store.get_global_account_data_by_type_for_user(
+            user_id, data_type
+        )
+        # We clone and freeze to prevent the module accidentally mutating the
+        # dict that lives in the cache, as that could introduce nasty bugs.
+        return freeze(data)
+
+    async def put_global(
+        self, user_id: str, data_type: str, new_data: JsonDict
+    ) -> None:
+        """
+        Puts some global account data, of a specified type, for the specified user.
+
+        The provided user ID must be a valid user ID of a local user.
+
+        Please note that this will overwrite existing the account data of that type
+        for that user!
+
+        Added in Synapse v1.57.0.
+        """
+        self._validate_user_id(user_id)
+
+        if not isinstance(data_type, str):
+            raise TypeError(f"data_type must be a str; got {type(data_type).__name__}")
+
+        if not isinstance(new_data, dict):
+            raise TypeError(f"new_data must be a dict; got {type(new_data).__name__}")
+
+        # Ensure the user exists, so we don't just write to users that aren't there.
+        if await self._store.get_userinfo_by_id(user_id) is None:
+            raise ValueError(f"User {user_id} does not exist on this server.")
+
+        await self._handler.add_account_data_for_user(user_id, data_type, new_data)
diff --git a/tests/module_api/test_account_data_manager.py b/tests/module_api/test_account_data_manager.py
new file mode 100644
index 000000000000..bec018d9e702
--- /dev/null
+++ b/tests/module_api/test_account_data_manager.py
@@ -0,0 +1,157 @@
+# 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 synapse.api.errors import SynapseError
+from synapse.rest import admin
+
+from tests.unittest import HomeserverTestCase
+
+
+class ModuleApiTestCase(HomeserverTestCase):
+    servlets = [
+        admin.register_servlets,
+    ]
+
+    def prepare(self, reactor, clock, homeserver) -> None:
+        self._store = homeserver.get_datastores().main
+        self._module_api = homeserver.get_module_api()
+        self._account_data_mgr = self._module_api.account_data_manager
+
+        self.user_id = self.register_user("kristina", "secret")
+
+    def test_get_global(self) -> None:
+        """
+        Tests that getting global account data through the module API works as
+        expected, including getting `None` for unset account data.
+        """
+        self.get_success(
+            self._store.add_account_data_for_user(
+                self.user_id, "test.data", {"wombat": True}
+            )
+        )
+
+        # Getting existent account data works as expected.
+        self.assertEqual(
+            self.get_success(
+                self._account_data_mgr.get_global(self.user_id, "test.data")
+            ),
+            {"wombat": True},
+        )
+
+        # Getting non-existent account data returns None.
+        self.assertIsNone(
+            self.get_success(
+                self._account_data_mgr.get_global(self.user_id, "no.data.at.all")
+            )
+        )
+
+    def test_get_global_validation(self) -> None:
+        """
+        Tests that invalid or remote user IDs are treated as errors and raised as exceptions,
+        whilst getting global account data for a user.
+
+        This is a design choice to try and communicate potential bugs to modules
+        earlier on.
+        """
+        with self.assertRaises(SynapseError):
+            self.get_success_or_raise(
+                self._account_data_mgr.get_global("this isn't a user id", "test.data")
+            )
+
+        with self.assertRaises(ValueError):
+            self.get_success_or_raise(
+                self._account_data_mgr.get_global("@valid.but:remote", "test.data")
+            )
+
+    def test_get_global_no_mutability(self) -> None:
+        """
+        Tests that modules can't introduce bugs into Synapse by mutating the result
+        of `get_global`.
+        """
+        # First add some account data to set up the test.
+        self.get_success(
+            self._store.add_account_data_for_user(
+                self.user_id, "test.data", {"wombat": True}
+            )
+        )
+
+        # Now request that data and then mutate it (out of negligence or otherwise).
+        the_data = self.get_success(
+            self._account_data_mgr.get_global(self.user_id, "test.data")
+        )
+        with self.assertRaises(TypeError):
+            # This throws an exception because it's a frozen dict.
+            the_data["wombat"] = False
+
+    def test_put_global(self) -> None:
+        """
+        Tests that written account data using `put_global` can be read out again later.
+        """
+
+        self.get_success(
+            self._module_api.account_data_manager.put_global(
+                self.user_id, "test.data", {"wombat": True}
+            )
+        )
+
+        # Request that account data from the normal store; check it's as we expect.
+        self.assertEqual(
+            self.get_success(
+                self._store.get_global_account_data_by_type_for_user(
+                    self.user_id, "test.data"
+                )
+            ),
+            {"wombat": True},
+        )
+
+    def test_put_global_validation(self) -> None:
+        """
+        Tests that a module can't write account data to user IDs that don't have
+        actual users registered to them.
+        Modules also must supply the correct types.
+        """
+
+        with self.assertRaises(SynapseError):
+            self.get_success_or_raise(
+                self._account_data_mgr.put_global(
+                    "this isn't a user id", "test.data", {}
+                )
+            )
+
+        with self.assertRaises(ValueError):
+            self.get_success_or_raise(
+                self._account_data_mgr.put_global("@valid.but:remote", "test.data", {})
+            )
+
+        with self.assertRaises(ValueError):
+            self.get_success_or_raise(
+                self._module_api.account_data_manager.put_global(
+                    "@notregistered:test", "test.data", {}
+                )
+            )
+
+        with self.assertRaises(TypeError):
+            # The account data type must be a string.
+            self.get_success_or_raise(
+                self._module_api.account_data_manager.put_global(
+                    self.user_id, 42, {}  # type: ignore
+                )
+            )
+
+        with self.assertRaises(TypeError):
+            # The account data dict must be a dict.
+            self.get_success_or_raise(
+                self._module_api.account_data_manager.put_global(
+                    self.user_id, "test.data", 42  # type: ignore
+                )
+            )