diff --git a/RELEASE-NOTES-1.37 b/RELEASE-NOTES-1.37
index 91373e79db3..e1b53252ecb 100644
--- a/RELEASE-NOTES-1.37
+++ b/RELEASE-NOTES-1.37
@@ -675,6 +675,8 @@ because of Phabricator reports.
   BaseTemplateAfterPortlet hook, which were deprecated in 1.35,
   now emit deprecation warnings.
 * The LocalFile::getHistory() hook is deprecated.
+* Previously the Skin templateDirectory option inside skin.json had to be
+  relative to MediaWiki core. This should now be relative to the skin.
 * Calling WikiPage::prepareContentForEdit() without a UserIdentity is now
   deprecated.
 * User::getEditTokenObject(), ::getEditToken(), and ::matchEditToken() were each
diff --git a/includes/registration/ExtensionProcessor.php b/includes/registration/ExtensionProcessor.php
index b6b4212b976..92982763cf4 100644
--- a/includes/registration/ExtensionProcessor.php
+++ b/includes/registration/ExtensionProcessor.php
@@ -638,8 +638,11 @@ class ExtensionProcessor implements Processor {
 					if ( is_dir( $correctedPath ) ) {
 						$data['args'][0]['templateDirectory'] = $correctedPath;
 					} else {
-						// TODO: deprecate directories relative to core.
 						$data['args'][0]['templateDirectory'] = $templateDirectory;
+						wfDeprecatedMsg(
+							'Template directory should be relative to skin or omitted.',
+							'1.37'
+						);
 					}
 				} elseif ( isset( $data['args'][0] ) ) {
 					// If not set, we set a sensible default.
diff --git a/tests/phpunit/includes/registration/ExtensionProcessorTest.php b/tests/phpunit/includes/registration/ExtensionProcessorTest.php
index 9fe702f2343..d1c675f9b23 100644
--- a/tests/phpunit/includes/registration/ExtensionProcessorTest.php
+++ b/tests/phpunit/includes/registration/ExtensionProcessorTest.php
@@ -63,6 +63,7 @@ class ExtensionProcessorTest extends MediaWikiIntegrationTestCase {
 	}
 
 	public function testExtractSkins() {
+		$this->expectDeprecation();
 		$processor = new ExtensionProcessor();
 		$processor->extractInfo( $this->dir, self::$default + [
 			'ValidSkinNames' => [