diff --git a/readthedocs/projects/forms.py b/readthedocs/projects/forms.py
index 2c0822b1b5a..3287b847def 100644
--- a/readthedocs/projects/forms.py
+++ b/readthedocs/projects/forms.py
@@ -818,8 +818,15 @@ def clean_domain(self):
 
         domain_string = parsed.netloc
 
-        # Don't allow production or public domain to be set as custom domain
-        for invalid_domain in [settings.PRODUCTION_DOMAIN, settings.PUBLIC_DOMAIN]:
+        # Don't allow internal domains to be added, we have:
+        # - Dashboard domain
+        # - Public domain (from where documentation pages are served)
+        # - External version domain (from where PR previews are served)
+        for invalid_domain in [
+            settings.PRODUCTION_DOMAIN,
+            settings.PUBLIC_DOMAIN,
+            settings.RTD_EXTERNAL_VERSION_DOMAIN,
+        ]:
             if invalid_domain and domain_string.endswith(invalid_domain):
                 raise forms.ValidationError(
                     f'{invalid_domain} is not a valid domain.'
diff --git a/readthedocs/rtd_tests/tests/test_domains.py b/readthedocs/rtd_tests/tests/test_domains.py
index 49c3843b16d..2af9ab831f7 100644
--- a/readthedocs/rtd_tests/tests/test_domains.py
+++ b/readthedocs/rtd_tests/tests/test_domains.py
@@ -92,6 +92,18 @@ def test_public_domain_not_allowed(self):
             f"{settings.PUBLIC_DOMAIN} is not a valid domain.",
         )
 
+    @override_settings(RTD_EXTERNAL_VERSION_DOMAIN="readthedocs.build")
+    def test_external_domain_not_allowed(self):
+        for domain in ["readthedocs.build", "test.readthedocs.build"]:
+            form = DomainForm(
+                {"domain": domain},
+                project=self.project,
+            )
+            self.assertFalse(form.is_valid())
+            self.assertEqual(
+                form.errors["domain"][0], "readthedocs.build is not a valid domain."
+            )
+
     def test_domain_with_path(self):
         form = DomainForm(
             {"domain": "domain.com/foo/bar"},