diff --git a/changelog.d/7908.feature b/changelog.d/7908.feature
new file mode 100644
index 0000000000..4b9a8d8569
--- /dev/null
+++ b/changelog.d/7908.feature
@@ -0,0 +1 @@
+Add the ability to re-activate an account from the admin API.
diff --git a/synapse/handlers/deactivate_account.py b/synapse/handlers/deactivate_account.py
index 696d85b5f9..25169157c1 100644
--- a/synapse/handlers/deactivate_account.py
+++ b/synapse/handlers/deactivate_account.py
@@ -30,6 +30,7 @@ class DeactivateAccountHandler(BaseHandler):
 
     def __init__(self, hs):
         super(DeactivateAccountHandler, self).__init__(hs)
+        self.hs = hs
         self._auth_handler = hs.get_auth_handler()
         self._device_handler = hs.get_device_handler()
         self._room_member_handler = hs.get_room_member_handler()
@@ -222,13 +223,26 @@ class DeactivateAccountHandler(BaseHandler):
         """
         Activate an account that was previously deactivated.
 
-        This simply marks the user as activate in the database and does not
-        attempt to rejoin rooms, re-add threepids, etc.
+        This marks the user as active and not erased in the database, but does
+        not attempt to rejoin rooms, re-add threepids, etc.
+
+        If enabled, the user will be re-added to the user directory.
 
         The user will also need a password hash set to actually login.
 
         Args:
-            user_id: ID of user to be deactivated
+            user_id: ID of user to be re-activated
         """
-        # Mark the user as activate.
+        # Add the user to the directory, if necessary.
+        user = UserID.from_string(user_id)
+        if self.hs.config.user_directory_search_all_users:
+            profile = await self.store.get_profileinfo(user.localpart)
+            await self.user_directory_handler.handle_local_profile_change(
+                user_id, profile
+            )
+
+        # Ensure the user is not marked as erased.
+        await self.store.mark_user_not_erased(user_id)
+
+        # Mark the user as active.
         await self.store.set_user_deactivated_status(user_id, False)
diff --git a/synapse/storage/data_stores/main/user_erasure_store.py b/synapse/storage/data_stores/main/user_erasure_store.py
index ec6b8a4ffd..d3038ff06d 100644
--- a/synapse/storage/data_stores/main/user_erasure_store.py
+++ b/synapse/storage/data_stores/main/user_erasure_store.py
@@ -70,11 +70,11 @@ class UserErasureWorkerStore(SQLBaseStore):
 
 
 class UserErasureStore(UserErasureWorkerStore):
-    def mark_user_erased(self, user_id):
+    def mark_user_erased(self, user_id: str) -> None:
         """Indicate that user_id wishes their message history to be erased.
 
         Args:
-            user_id (str): full user_id to be erased
+            user_id: full user_id to be erased
         """
 
         def f(txn):
@@ -89,3 +89,25 @@ class UserErasureStore(UserErasureWorkerStore):
             self._invalidate_cache_and_stream(txn, self.is_user_erased, (user_id,))
 
         return self.db.runInteraction("mark_user_erased", f)
+
+    def mark_user_not_erased(self, user_id: str) -> None:
+        """Indicate that user_id is no longer erased.
+
+        Args:
+            user_id: full user_id to be un-erased
+        """
+
+        def f(txn):
+            # first check if they are already in the list
+            txn.execute("SELECT 1 FROM erased_users WHERE user_id = ?", (user_id,))
+            if not txn.fetchone():
+                return
+
+            # They are there, delete them.
+            self.simple_delete_one_txn(
+                txn, "erased_users", keyvalues={"user_id": user_id}
+            )
+
+            self._invalidate_cache_and_stream(txn, self.is_user_erased, (user_id,))
+
+        return self.db.runInteraction("mark_user_not_erased", f)