diff --git a/api_docs/changelog.md b/api_docs/changelog.md
index 9517b8ce82..f67c8da02c 100644
--- a/api_docs/changelog.md
+++ b/api_docs/changelog.md
@@ -20,6 +20,12 @@ format used by the Zulip server that they are interacting with.
 
 ## Changes in Zulip 10.0
 
+**Feature level 289**
+
+* [`POST /users/{user_id}/subscription`](/api/subscribe): In the response,
+  users are identified by their numeric user ID rather than by their
+  Zulip API email address.
+
 **Feature level 288**
 
 * [`POST /register`](/api/register-queue):
diff --git a/version.py b/version.py
index c41b8f6bbf..95b21659f6 100644
--- a/version.py
+++ b/version.py
@@ -34,7 +34,7 @@ DESKTOP_WARNING_VERSION = "5.9.3"
 # new level means in api_docs/changelog.md, as well as "**Changes**"
 # entries in the endpoint's documentation in `zulip.yaml`.
 
-API_FEATURE_LEVEL = 288  # Last bumped for presence_history_limit_days
+API_FEATURE_LEVEL = 289  # return ID as key while subscribing to a stream.
 
 # Bump the minor PROVISION_VERSION to indicate that folks should provision
 # only when going from an old version of the code to a newer version. Bump
diff --git a/web/src/stream_edit_subscribers.ts b/web/src/stream_edit_subscribers.ts
index b38f69b3ef..cba3fd1a5a 100644
--- a/web/src/stream_edit_subscribers.ts
+++ b/web/src/stream_edit_subscribers.ts
@@ -226,11 +226,11 @@ function subscribe_new_users({pill_user_ids}: {pill_user_ids: number[]}): void {
     function invite_success(raw_data: unknown): void {
         const data = add_user_ids_api_response_schema.parse(raw_data);
         pill_widget.clear();
-        const subscribed_users = Object.keys(data.subscribed).map(
-            (email) => people.get_by_email(email)!,
+        const subscribed_users = Object.keys(data.subscribed).map((user_id) =>
+            people.get_by_user_id(Number(user_id)),
         );
-        const already_subscribed_users = Object.keys(data.already_subscribed).map(
-            (email) => people.get_by_email(email)!,
+        const already_subscribed_users = Object.keys(data.already_subscribed).map((user_id) =>
+            people.get_by_user_id(Number(user_id)),
         );
 
         show_stream_subscription_request_result({
diff --git a/zerver/openapi/python_examples.py b/zerver/openapi/python_examples.py
index cca31a7580..479c0218f1 100644
--- a/zerver/openapi/python_examples.py
+++ b/zerver/openapi/python_examples.py
@@ -129,7 +129,7 @@ def add_subscriptions(client: Client) -> None:
     # {code_example|end}
     assert_success_response(result)
     validate_against_openapi_schema(result, "/users/me/subscriptions", "post", "200")
-    assert "newbie@zulip.com" in result["subscribed"]
+    assert str(user_id) in result["subscribed"]
 
 
 def test_add_subscriptions_already_subscribed(client: Client) -> None:
diff --git a/zerver/openapi/zulip.yaml b/zerver/openapi/zulip.yaml
index 833f28cba8..b25f9a4650 100644
--- a/zerver/openapi/zulip.yaml
+++ b/zerver/openapi/zulip.yaml
@@ -9916,12 +9916,15 @@ paths:
                       subscribed:
                         type: object
                         description: |
-                          A dictionary where the key is the Zulip API email address of the user/bot
-                          and the value is a list of the names of the channels that were subscribed
-                          to as a result of the query.
+                          A dictionary where the key is the ID of the user and the value
+                          is a list of the names of the channels that user was subscribed
+                          to as a result of the request.
+
+                          **Changes**: Before Zulip 10.0 (feature level 289), the user
+                          keys were Zulip API email addresses, not user ID.
                         additionalProperties:
                           description: |
-                            `{email_address}`: List of the names of the channels that were subscribed
+                            `{id}`: List of the names of the channels that were subscribed
                             to as a result of the query.
                           type: array
                           items:
@@ -9929,12 +9932,16 @@ paths:
                       already_subscribed:
                         type: object
                         description: |
-                          A dictionary where the key is the Zulip API email address of the user/bot
-                          and the value is a list of the names of the channels that the user/bot is
-                          already subscribed to.
+                          A dictionary where the key is the ID of the user and the value
+                          is a list of the names of the channels that where the user was
+                          not added as a subscriber in this request, because they were
+                          already a subscriber.
+
+                          **Changes**: Before Zulip 10.0 (feature level 289), the user
+                          keys were Zulip API email addresses, not user IDs.
                         additionalProperties:
                           description: |
-                            `{email_address}`: List of the names of the channels that the user is
+                            `{id}`: List of the names of the channels that the user is
                             already subscribed to.
                           type: array
                           items:
@@ -9948,11 +9955,10 @@ paths:
                           authorized to subscribe to. Only present if `"authorization_errors_fatal": false`.
                     example:
                       {
-                        "already_subscribed":
-                          {"iago@zulip.com": ["testing-help"]},
+                        "already_subscribed": {"1": ["testing-help"]},
                         "msg": "",
                         "result": "success",
-                        "subscribed": {"newbie@zulip.com": ["testing-help"]},
+                        "subscribed": {"2": ["testing-help"]},
                       }
         "400":
           description: Bad request.
diff --git a/zerver/tests/test_subs.py b/zerver/tests/test_subs.py
index 9c52cfc540..72f017e3ac 100644
--- a/zerver/tests/test_subs.py
+++ b/zerver/tests/test_subs.py
@@ -495,10 +495,10 @@ class TestCreateStreams(ZulipTestCase):
             "result": "success",
             "msg": "",
             "subscribed": {
-                "AARON@zulip.com": ["brand new stream"],
-                "cordelia@zulip.com": ["brand new stream"],
-                "hamlet@zulip.com": ["brand new stream"],
-                "iago@zulip.com": ["brand new stream"],
+                "10": ["brand new stream"],
+                "11": ["brand new stream"],
+                "6": ["brand new stream"],
+                "8": ["brand new stream"],
             },
             "already_subscribed": {},
         }
@@ -4062,6 +4062,7 @@ class SubscriptionAPITest(ZulipTestCase):
         self.user_profile = self.example_user("hamlet")
         self.test_email = self.user_profile.email
         self.test_user = self.user_profile
+        self.test_id = str(self.user_profile.id)
         self.login_user(self.user_profile)
         self.test_realm = self.user_profile.realm
         self.streams = self.get_streams(self.user_profile)
@@ -4170,29 +4171,30 @@ class SubscriptionAPITest(ZulipTestCase):
         email: str,
         new_subs: list[str],
         realm: Realm,
+        id: str,
         invite_only: bool = False,
     ) -> None:
         """
         Check result of adding subscriptions.
 
         You can add subscriptions for yourself or possibly many
-        principals, which is why e-mails map to subscriptions in the
+        principals, which is why user ID map to subscriptions in the
         result.
 
         The result json is of the form
 
         {"msg": "",
          "result": "success",
-         "already_subscribed": {self.example_email("iago"): ["Venice", "Verona"]},
-         "subscribed": {self.example_email("iago"): ["Venice8"]}}
+         "already_subscribed": {self.example_user("iago").id: ["Venice", "Verona"]},
+         "subscribed": {self.example_user("iago").id: ["Venice8"]}}
         """
         result = self.common_subscribe_to_streams(
             self.test_user, subscriptions, other_params, invite_only=invite_only
         )
         json = self.assert_json_success(result)
-        self.assertEqual(sorted(subscribed), sorted(json["subscribed"][email]))
-        self.assertEqual(sorted(already_subscribed), sorted(json["already_subscribed"][email]))
-        user = get_user(email, realm)
+        self.assertEqual(sorted(subscribed), sorted(json["subscribed"][id]))
+        self.assertEqual(sorted(already_subscribed), sorted(json["already_subscribed"][id]))
+        user = get_user_profile_by_id_in_realm(int(id), realm)
         new_streams = self.get_streams(user)
         self.assertEqual(sorted(new_streams), sorted(new_subs))
 
@@ -4217,6 +4219,7 @@ class SubscriptionAPITest(ZulipTestCase):
                 self.test_email,
                 self.streams + add_streams,
                 self.test_realm,
+                self.test_id,
             )
 
     def test_successful_subscriptions_add_with_announce(self) -> None:
@@ -4246,6 +4249,7 @@ class SubscriptionAPITest(ZulipTestCase):
                 self.test_email,
                 self.streams + add_streams,
                 self.test_realm,
+                self.test_id,
             )
 
         expected_stream_ids = {get_stream(stream, self.test_realm).id for stream in add_streams}
@@ -4392,6 +4396,7 @@ class SubscriptionAPITest(ZulipTestCase):
             self.test_email,
             [*self.streams, "hümbüǵ"],
             self.test_realm,
+            self.test_id,
         )
 
     def test_subscriptions_add_too_long(self) -> None:
@@ -4703,6 +4708,7 @@ class SubscriptionAPITest(ZulipTestCase):
             other_profile.email,
             streams_to_sub,
             invitee_realm,
+            str(other_profile.id),
             invite_only=invite_only,
         )
 
@@ -4875,7 +4881,7 @@ class SubscriptionAPITest(ZulipTestCase):
         do_change_stream_post_policy(stream, Stream.STREAM_POST_POLICY_ADMINS, acting_user=member)
         result = self.common_subscribe_to_streams(member, ["stream1"])
         json = self.assert_json_success(result)
-        self.assertEqual(json["subscribed"], {member.email: ["stream1"]})
+        self.assertEqual(json["subscribed"], {str(member.id): ["stream1"]})
         self.assertEqual(json["already_subscribed"], {})
 
     def test_subscribe_to_stream_post_policy_restrict_new_members_stream(self) -> None:
@@ -4895,7 +4901,7 @@ class SubscriptionAPITest(ZulipTestCase):
         )
         result = self.common_subscribe_to_streams(new_member, ["stream1"])
         json = self.assert_json_success(result)
-        self.assertEqual(json["subscribed"], {new_member.email: ["stream1"]})
+        self.assertEqual(json["subscribed"], {str(new_member.id): ["stream1"]})
         self.assertEqual(json["already_subscribed"], {})
 
     def test_subscribe_to_stream_post_policy_moderators_stream(self) -> None:
@@ -4912,7 +4918,7 @@ class SubscriptionAPITest(ZulipTestCase):
         )
         result = self.common_subscribe_to_streams(member, ["stream1"])
         json = self.assert_json_success(result)
-        self.assertEqual(json["subscribed"], {member.email: ["stream1"]})
+        self.assertEqual(json["subscribed"], {str(member.id): ["stream1"]})
         self.assertEqual(json["already_subscribed"], {})
 
     def test_guest_user_subscribe(self) -> None:
@@ -5897,7 +5903,7 @@ class InviteOnlyStreamTest(ZulipTestCase):
         result = self.common_subscribe_to_streams(hamlet, [stream_name], invite_only=True)
 
         json = self.assert_json_success(result)
-        self.assertEqual(json["subscribed"], {hamlet.email: [stream_name]})
+        self.assertEqual(json["subscribed"], {str(hamlet.id): [stream_name]})
         self.assertEqual(json["already_subscribed"], {})
 
         # Subscribing oneself to an invite-only stream is not allowed
@@ -5925,7 +5931,7 @@ class InviteOnlyStreamTest(ZulipTestCase):
             extra_post_data={"principals": orjson.dumps([othello.id]).decode()},
         )
         json = self.assert_json_success(result)
-        self.assertEqual(json["subscribed"], {othello.email: [stream_name]})
+        self.assertEqual(json["subscribed"], {str(othello.id): [stream_name]})
         self.assertEqual(json["already_subscribed"], {})
 
         # Make sure both users are subscribed to this stream
diff --git a/zerver/views/streams.py b/zerver/views/streams.py
index 9d83a8ef28..70a0f02819 100644
--- a/zerver/views/streams.py
+++ b/zerver/views/streams.py
@@ -664,9 +664,7 @@ def add_subscriptions_backend(
         realm, streams, subscribers, acting_user=user_profile, color_map=color_map
     )
 
-    # We can assume unique emails here for now, but we should eventually
-    # convert this function to be more id-centric.
-    email_to_user_profile: dict[str, UserProfile] = {}
+    id_to_user_profile: dict[str, UserProfile] = {}
 
     result: dict[str, Any] = dict(
         subscribed=defaultdict(list), already_subscribed=defaultdict(list)
@@ -674,12 +672,14 @@ def add_subscriptions_backend(
     for sub_info in subscribed:
         subscriber = sub_info.user
         stream = sub_info.stream
-        result["subscribed"][subscriber.email].append(stream.name)
-        email_to_user_profile[subscriber.email] = subscriber
+        user_id = str(subscriber.id)
+        result["subscribed"][user_id].append(stream.name)
+        id_to_user_profile[user_id] = subscriber
     for sub_info in already_subscribed:
         subscriber = sub_info.user
         stream = sub_info.stream
-        result["already_subscribed"][subscriber.email].append(stream.name)
+        user_id = str(subscriber.id)
+        result["already_subscribed"][user_id].append(stream.name)
 
     result["subscribed"] = dict(result["subscribed"])
     result["already_subscribed"] = dict(result["already_subscribed"])
@@ -688,7 +688,7 @@ def add_subscriptions_backend(
         user_profile=user_profile,
         subscribers=subscribers,
         new_subscriptions=result["subscribed"],
-        email_to_user_profile=email_to_user_profile,
+        id_to_user_profile=id_to_user_profile,
         created_streams=created_streams,
         announce=announce,
     )
@@ -704,7 +704,7 @@ def send_messages_for_new_subscribers(
     user_profile: UserProfile,
     subscribers: set[UserProfile],
     new_subscriptions: dict[str, list[str]],
-    email_to_user_profile: dict[str, UserProfile],
+    id_to_user_profile: dict[str, UserProfile],
     created_streams: list[Stream],
     announce: bool,
 ) -> None:
@@ -716,7 +716,7 @@ def send_messages_for_new_subscribers(
     excessive query counts by mocking this function so that it
     doesn't drown out query counts from other code.
     """
-    bots = {subscriber.email: subscriber.is_bot for subscriber in subscribers}
+    bots = {str(subscriber.id): subscriber.is_bot for subscriber in subscribers}
 
     newly_created_stream_names = {s.name for s in created_streams}
 
@@ -727,11 +727,11 @@ def send_messages_for_new_subscribers(
     # or if a new stream was created with the "announce" option.
     notifications = []
     if new_subscriptions:
-        for email, subscribed_stream_names in new_subscriptions.items():
-            if email == user_profile.email:
+        for id, subscribed_stream_names in new_subscriptions.items():
+            if id == str(user_profile.id):
                 # Don't send a Zulip if you invited yourself.
                 continue
-            if bots[email]:
+            if bots[id]:
                 # Don't send invitation Zulips to bots
                 continue
 
@@ -742,7 +742,7 @@ def send_messages_for_new_subscribers(
             if not notify_stream_names:
                 continue
 
-            recipient_user = email_to_user_profile[email]
+            recipient_user = id_to_user_profile[id]
             sender = get_system_bot(settings.NOTIFICATION_BOT, recipient_user.realm_id)
 
             msg = you_were_just_subscribed_message(