diff --git a/CHANGELOG.txt b/CHANGELOG.txt
index ade6928dc61..671b400f454 100644
--- a/CHANGELOG.txt
+++ b/CHANGELOG.txt
@@ -76,6 +76,7 @@ Changelog
  * Fix: Avoid showing scrollbars in the block picker unless necessary (Babitha Kumari)
  * Fix: Always show Add buttons, guide lines, Move up/down, Duplicate, Delete; in StreamField and Inline Panel (Thibaud Colas)
  * Fix: Make admin JS i18n endpoint accessible to non-authenticated users (Matt Westcott)
+ * Fix: Fix incorrect API serialisation for document `download_url` when `WAGTAILDOCS_SERVE_METHOD` is `direct` (Swojak-A)
  * Autosize text area field will now correctly resize when switching between comments toggle states (Suyash Srivastava)
  * Docs: Add code block to make it easier to understand contribution docs (Suyash Singh)
  * Docs: Add new "Icons" page for icons customisation and reuse across the admin interface (Coen van der Kamp, Thibaud Colas)
diff --git a/CONTRIBUTORS.rst b/CONTRIBUTORS.rst
index d9fe50dff3a..8a920d8bb02 100644
--- a/CONTRIBUTORS.rst
+++ b/CONTRIBUTORS.rst
@@ -706,6 +706,7 @@ Contributors
 * Mansi Gundre
 * Hanoon
 * Steve Steinwand
+* Swojak-A
 
 Translators
 ===========
diff --git a/docs/releases/5.0.md b/docs/releases/5.0.md
index 1c77a0ce2bf..738888c8a3c 100644
--- a/docs/releases/5.0.md
+++ b/docs/releases/5.0.md
@@ -118,6 +118,7 @@ Those improvements were implemented by Albina Starykova as part of an [Outreachy
  * Always show Add buttons, guide lines, Move up/down, Duplicate, Delete; in StreamField and Inline Panel (Thibaud Colas)
  * Make admin JS i18n endpoint accessible to non-authenticated users (Matt Westcott)
  * Autosize text area field will now correctly resize when switching between comments toggle states (Suyash Srivastava)
+ * Fix incorrect API serialisation for document `download_url` when `WAGTAILDOCS_SERVE_METHOD` is `direct` (Swojak-A)
 
 ### Documentation
 
diff --git a/wagtail/api/v2/utils.py b/wagtail/api/v2/utils.py
index c9f91034f83..e88d89e4cdd 100644
--- a/wagtail/api/v2/utils.py
+++ b/wagtail/api/v2/utils.py
@@ -27,6 +27,8 @@ def get_base_url(request=None):
 
 
 def get_full_url(request, path):
+    if path.startswith(("http://", "https://")):
+        return path
     base_url = get_base_url(request) or ""
     return base_url + path
 
diff --git a/wagtail/documents/tests/test_serializers.py b/wagtail/documents/tests/test_serializers.py
new file mode 100644
index 00000000000..cf2e9b48fb2
--- /dev/null
+++ b/wagtail/documents/tests/test_serializers.py
@@ -0,0 +1,59 @@
+from django.core.files.base import ContentFile
+from django.test import TestCase
+from django.test.utils import override_settings
+from django.urls import reverse
+
+from wagtail.documents import models
+
+
+class TestCorrectDownloadUrlSerialization(TestCase):
+
+    """Test asserts that in case of both `redirect` and `direct`
+    WAGTAILDOCS_SERVE_METHOD settings `download_url` field
+    is correctly serialized by DocumentDownloadUrlField."""
+
+    def setUp(self):
+        self.document = models.Document(title="Test document", file_hash="123456")
+        self.document.file.save("example.doc", ContentFile("A boring example document"))
+
+    def tearDown(self):
+        # delete the FieldFile directly because the TestCase does not commit
+        # transactions to trigger transaction.on_commit() in the signal handler
+        self.document.file.delete()
+
+    def get_response(self, document_id, **params):
+        return self.client.get(
+            reverse("wagtailapi_v2:documents:detail", args=(document_id,)), params
+        )
+
+    @override_settings(
+        WAGTAILDOCS_SERVE_METHOD="redirect",
+        DEFAULT_FILE_STORAGE="wagtail.test.dummy_external_storage.DummyExternalStorage",
+        WAGTAILAPI_BASE_URL="http://example.com/",
+    )
+    def test_serializer_wagtaildocs_serve_redirect(self):
+        response = self.get_response(self.document.id)
+        data = response.json()
+        self.assertIn("meta", data)
+        meta = data["meta"]
+        self.assertIn("download_url", meta)
+        download_url = meta["download_url"]
+        expected_url = "http://example.com/documents/%d/example.doc" % self.document.pk
+        self.assertEqual(download_url, expected_url)
+
+    @override_settings(
+        WAGTAILDOCS_SERVE_METHOD="direct",
+        DEFAULT_FILE_STORAGE="wagtail.test.dummy_external_storage.DummyExternalStorage",
+        MEDIA_URL="http://remotestorage.com/media/",
+        WAGTAILAPI_BASE_URL="http://example.com/",
+    )
+    def test_serializer_wagtaildocs_serve_direct(self):
+        response = self.get_response(self.document.id)
+        data = response.json()
+        self.assertIn("meta", data)
+        meta = data["meta"]
+        self.assertIn("download_url", meta)
+        download_url = meta["download_url"]
+        self.assertEqual(
+            download_url, "http://remotestorage.com/media/documents/example.doc"
+        )