Index: class-wp-rest-posts-controller.php
===================================================================
--- class-wp-rest-posts-controller.php	(revision 45491)
+++ class-wp-rest-posts-controller.php	(working copy)
@@ -2224,7 +2224,20 @@
 			$schema['links'] = $schema_links;
 		}
 
-		return $this->add_additional_fields_schema( $schema );
+		$schema = $this->add_additional_fields_schema( $schema );
+
+		/**
+		 * Filter the post's schema.
+		 *
+		 * The dynamic part of the filter `$this->post_type` refers to the post
+		 * type slug for the controller.
+		 *
+		 * @since 5.3.0
+		 *
+		 * @param array  $schema          Item schema data.
+		 * @param string $this->post_type Post type slug.
+		 */
+		return apply_filters( "rest_{$this->post_type}_item_schema", $schema, $this->post_type );
 	}
 
 	/**