diff --git a/readthedocs/api/v3/mixins.py b/readthedocs/api/v3/mixins.py
index 02be261b1e9..9db1091600a 100644
--- a/readthedocs/api/v3/mixins.py
+++ b/readthedocs/api/v3/mixins.py
@@ -94,8 +94,9 @@ def _get_parent_project(self):
         return get_object_or_404(Project, slug=slug)
 
     def _get_parent_build(self):
-        pk = self._get_parent_object_lookup(self.BUILD_LOOKUP_NAMES)
-        return get_object_or_404(Build, pk=pk)
+        project_slug = self._get_parent_object_lookup(self.PROJECT_LOOKUP_NAMES)
+        build_pk = self._get_parent_object_lookup(self.BUILD_LOOKUP_NAMES)
+        return get_object_or_404(Build, pk=build_pk, project__slug=project_slug)
 
     def _get_parent_version(self):
         project_slug = self._get_parent_object_lookup(self.PROJECT_LOOKUP_NAMES)
diff --git a/readthedocs/api/v3/tests/mixins.py b/readthedocs/api/v3/tests/mixins.py
index c1fb1de7dff..739008bcd2f 100644
--- a/readthedocs/api/v3/tests/mixins.py
+++ b/readthedocs/api/v3/tests/mixins.py
@@ -12,7 +12,7 @@
 from rest_framework.authtoken.models import Token
 from rest_framework.test import APIClient
 
-from readthedocs.builds.constants import TAG
+from readthedocs.builds.constants import LATEST, TAG
 from readthedocs.builds.models import Build, Version
 from readthedocs.core.notifications import MESSAGE_EMAIL_VALIDATION_PENDING
 from readthedocs.doc_builder.exceptions import BuildCancelled
@@ -98,6 +98,7 @@ def setUp(self):
 
         self.build = fixture.get(
             Build,
+            id=1,
             date=self.created,
             type="html",
             state="finished",
@@ -125,6 +126,21 @@ def setUp(self):
             external_builds_privacy_level=PUBLIC,
             privacy_level=PUBLIC,
         )
+        self.others_version = self.others_project.versions.get(slug=LATEST)
+        self.others_build = fixture.get(
+            Build,
+            date=self.created,
+            type="html",
+            state="finished",
+            error="",
+            success=True,
+            _config={"property": "test value"},
+            version=self.others_version,
+            project=self.others_project,
+            builder="builder01",
+            commit="a1b2c3",
+            length=60,
+        )
 
         # Make all non-html true so responses are complete
         self.project.versions.update(
@@ -171,6 +187,13 @@ def setUp(self):
             message_id=MESSAGE_EMAIL_VALIDATION_PENDING,
         )
 
+        self.notification_others_build = fixture.get(
+            Notification,
+            attached_to_content_type=ContentType.objects.get_for_model(Build),
+            attached_to_id=self.others_build.pk,
+            message_id=BuildCancelled.CANCELLED_BY_USER,
+        )
+
         self.client = APIClient()
 
     def tearDown(self):
diff --git a/readthedocs/api/v3/tests/responses/projects-versions-builds-list_POST.json b/readthedocs/api/v3/tests/responses/projects-versions-builds-list_POST.json
index e1d05ba0a9a..d434936ee6c 100644
--- a/readthedocs/api/v3/tests/responses/projects-versions-builds-list_POST.json
+++ b/readthedocs/api/v3/tests/responses/projects-versions-builds-list_POST.json
@@ -5,15 +5,15 @@
         "duration": null,
         "error": "",
         "finished": null,
-        "id": 2,
+        "id": 3,
         "_links": {
-            "_self": "https://readthedocs.org/api/v3/projects/project/builds/2/",
-            "notifications": "https://readthedocs.org/api/v3/projects/project/builds/2/notifications/",
+            "_self": "https://readthedocs.org/api/v3/projects/project/builds/3/",
+            "notifications": "https://readthedocs.org/api/v3/projects/project/builds/3/notifications/",
             "project": "https://readthedocs.org/api/v3/projects/project/",
             "version": "https://readthedocs.org/api/v3/projects/project/versions/v1.0/"
         },
         "urls": {
-            "build": "https://readthedocs.org/projects/project/builds/2/",
+            "build": "https://readthedocs.org/projects/project/builds/3/",
             "project": "https://readthedocs.org/projects/project/",
             "version": "https://readthedocs.org/dashboard/project/version/v1.0/edit/"
         },
diff --git a/readthedocs/api/v3/tests/test_builds.py b/readthedocs/api/v3/tests/test_builds.py
index 2410d0c845e..abd1b89349d 100644
--- a/readthedocs/api/v3/tests/test_builds.py
+++ b/readthedocs/api/v3/tests/test_builds.py
@@ -149,6 +149,28 @@ def test_projects_builds_notifications_list_other_user(self):
         response = self.client.get(url)
         self.assertEqual(response.status_code, 403)
 
+        # User can see their own notifications.
+        url = reverse(
+            "projects-builds-notifications-list",
+            kwargs={
+                "parent_lookup_project__slug": self.others_project.slug,
+                "parent_lookup_build__id": self.others_build.pk,
+            },
+        )
+        response = self.client.get(url)
+        self.assertEqual(response.status_code, 200)
+
+        # User can't see notifications from other users through his project.
+        url = reverse(
+            "projects-builds-notifications-list",
+            kwargs={
+                "parent_lookup_project__slug": self.others_project.slug,
+                "parent_lookup_build__id": self.build.pk,
+            },
+        )
+        response = self.client.get(url)
+        self.assertEqual(response.status_code, 404)
+
     def test_projects_builds_notifications_list_post(self):
         url = reverse(
             "projects-builds-notifications-list",