diff --git a/testing.rst b/testing.rst
index 75b8d791972..7e87f116058 100644
--- a/testing.rst
+++ b/testing.rst
@@ -1034,6 +1034,9 @@ Mailer Assertions
     Asserts that the given address header equals the expected e-mail
     address. This assertion normalizes addresses like ``Jane Smith
     <jane@example.com>`` into ``jane@example.com``.
+``assertEmailSubjectContains(RawMessage $email, string $expectedValue, string $message = '')``/``assertEmailSubjectNotContains(RawMessage $email, string $expectedValue, string $message = '')``
+    Asserts that the subject of the given email does (not) contain the
+    expected subject.
 
 Notifier Assertions
 ...................