diff --git a/patchwork/tests/api/test_series.py b/patchwork/tests/api/test_series.py
index 597b61a9..801e9449 100644
--- a/patchwork/tests/api/test_series.py
+++ b/patchwork/tests/api/test_series.py
@@ -2,6 +2,7 @@
 # Copyright (C) 2018 Stephen Finucane <stephen@that.guru>
 #
 # SPDX-License-Identifier: GPL-2.0-or-later
+import json
 
 from django.test import override_settings
 from django.urls import NoReverseMatch
@@ -16,6 +17,7 @@
 from patchwork.tests.utils import create_project
 from patchwork.tests.utils import create_series
 from patchwork.tests.utils import create_user
+from patchwork.models import Person
 
 
 @override_settings(ENABLE_REST_API=True)
@@ -203,3 +205,75 @@ def test_create_update_delete(self):
 
         resp = self.client.delete(self.api_url(series.id))
         self.assertEqual(status.HTTP_405_METHOD_NOT_ALLOWED, resp.status_code)
+
+    def _generate_related_ids_payload(self, series, id):
+        related_ids = series.related_series.all().values_list('id', flat=True)
+        return list(related_ids) + [id]
+
+    def test_series_linking(self):
+        user = create_user()
+        person = Person.objects.get(user=user)
+        project_obj = create_project(linkname='myproject')
+        series_a = create_series(project=project_obj, submitter=person)
+        create_cover(series=series_a)
+        create_patch(series=series_a)
+
+        self.client.authenticate(user=user)
+        url = reverse('api-series-detail', kwargs={'pk': series_a.id})
+
+        # Link to another series
+        series_b = create_series(
+            project=series_a.project, submitter=series_a.submitter
+        )
+
+        resp = self.client.patch(
+            url,
+            data={
+                'related_series': self._generate_related_ids_payload(
+                    series_a, series_b.id
+                )
+            },
+        )
+        related_series = json.loads(resp.content).get('related_series')
+        self.assertEqual(resp.status_code, status.HTTP_200_OK)
+        self.assertEqual(len(related_series), 1)
+        self.assertEqual(
+            related_series[0]['web_url'],
+            f'http://example.com/project/myproject/list/?series={series_b.id}',
+        )
+
+        # Link to more than one series
+        series_c = create_series(
+            project=series_a.project, submitter=series_a.submitter
+        )
+        resp = self.client.patch(
+            url,
+            data={
+                'related_series': self._generate_related_ids_payload(
+                    series_a, series_c.id
+                )
+            },
+        )
+
+        related_series = json.loads(resp.content).get('related_series')
+        self.assertEqual(resp.status_code, status.HTTP_200_OK)
+        self.assertEqual(len(related_series), 2)
+        self.assertEqual(
+            related_series[1]['web_url'],
+            f'http://example.com/project/myproject/list/?series={series_c.id}',
+        )
+
+        # Link to a series from a different project
+        series_d = create_series(submitter=series_a.submitter)
+
+        resp = self.client.patch(
+            url,
+            data={
+                'related_series': self._generate_related_ids_payload(
+                    series_a, series_d.id
+                )
+            },
+        )
+
+        related_series = json.loads(resp.content).get('related_series')
+        self.assertEqual(resp.status_code, status.HTTP_400_BAD_REQUEST)
diff --git a/patchwork/tests/test_series.py b/patchwork/tests/test_series.py
index ce114042..220cb81e 100644
--- a/patchwork/tests/test_series.py
+++ b/patchwork/tests/test_series.py
@@ -8,11 +8,13 @@
 import unittest
 
 from django.test import TestCase
+from rest_framework.request import HttpRequest
 
 from patchwork import models
 from patchwork import parser
 from patchwork.tests import utils
 from patchwork.views.utils import patch_to_mbox
+from patchwork.api.series import SeriesSerializer
 
 
 TEST_SERIES_DIR = os.path.join(os.path.dirname(__file__), 'series')
@@ -804,3 +806,49 @@ def test_custom_name(self):
         self.assertEqual(series.name, series_name)
 
         mbox.close()
+
+
+class SeriesSerializerTestCase(TestCase):
+    def _mock_request(self):
+        mock_request = HttpRequest()
+        mock_request.version = '1.4'
+        mock_request.META['SERVER_NAME'] = 'example.com'
+        mock_request.META['SERVER_PORT'] = '8000'
+
+        return mock_request
+
+    def _create_serializer(self, series, related_series, mock_request):
+        related_ids = list(
+            series.related_series.all().values_list('id', flat=True)
+        ) + [related_series.pk]
+
+        return SeriesSerializer(
+            series,
+            context={'request': mock_request},
+            data={'related_series': related_ids},
+            partial=True,
+        )
+
+    def test_related_series_validation_equal_project_id(self):
+        series_a = utils.create_series()
+        series_b = utils.create_series(project=series_a.project)
+
+        mock_request = self._mock_request()
+        serializer = self._create_serializer(series_a, series_b, mock_request)
+        serializer.is_valid()
+        serializer.save()
+
+        related_series_urls = serializer.data['related_series']
+        self.assertEqual(len(related_series_urls), 1)
+        self.assertEqual(
+            f'series={series_b.id}' in related_series_urls[0]['web_url'], True
+        )
+
+    def test_related_series_validation_different_project_id(self):
+        series_a = utils.create_series()
+        series_b = utils.create_series()
+
+        mock_request = self._mock_request()
+        serializer = self._create_serializer(series_a, series_b, mock_request)
+        is_valid = serializer.is_valid()
+        self.assertFalse(is_valid)