diff --git a/readthedocs/config/config.py b/readthedocs/config/config.py
index 38f30fa5174..a8db45e9418 100644
--- a/readthedocs/config/config.py
+++ b/readthedocs/config/config.py
@@ -268,9 +268,9 @@ def valid_build_images(self):
 
         The user can use any of this values in the YAML file. These values are
         the keys of ``DOCKER_IMAGE_SETTINGS`` Django setting (without the
-        ``readthedocs/build`` part) plus ``stable`` and ``latest``.
+        ``readthedocs/build`` part) plus ``stable``, ``latest`` and ``testing``.
         """
-        images = {'stable', 'latest'}
+        images = {'stable', 'latest', 'testing'}
         for k in settings.DOCKER_IMAGE_SETTINGS:
             _, version = k.split(':')
             if re.fullmatch(r'^[\d\.]+$', version):
diff --git a/readthedocs/config/tests/test_config.py b/readthedocs/config/tests/test_config.py
index a5b5907b887..4ea007ba288 100644
--- a/readthedocs/config/tests/test_config.py
+++ b/readthedocs/config/tests/test_config.py
@@ -880,7 +880,7 @@ def test_conda_check_file_required(self):
             build.validate()
         assert excinfo.value.key == 'conda.environment'
 
-    @pytest.mark.parametrize('value', ['stable', 'latest'])
+    @pytest.mark.parametrize('value', ['stable', 'latest', 'testing'])
     def test_build_image_check_valid(self, value):
         build = self.get_build_config({'build': {'image': value}})
         build.validate()