diff --git a/sites/default/default.settings.php b/sites/default/default.settings.php
index 0da98e8bb31..94a1e0478be 100644
--- a/sites/default/default.settings.php
+++ b/sites/default/default.settings.php
@@ -144,6 +144,11 @@
  * @code
  *   'prefix' => 'main_',
  * @endcode
+ *
+ * Per-table prefixes are deprecated as of Drupal 8.2, and will be removed in
+ * Drupal 9.0. After that, only a single prefix for all tables will be
+ * supported.
+ *
  * To provide prefixes for specific tables, set 'prefix' as an array.
  * The array's keys are the table names and the values are the prefixes.
  * The 'default' element is mandatory and holds the prefix for any tables