diff --git a/WordPress/Helpers/IsUnitTestTrait.php b/WordPress/Helpers/IsUnitTestTrait.php
index e4a0869b92..85439c40ee 100644
--- a/WordPress/Helpers/IsUnitTestTrait.php
+++ b/WordPress/Helpers/IsUnitTestTrait.php
@@ -73,12 +73,13 @@ trait IsUnitTestTrait {
 	 *           directory of WP Core.}
 	 *
 	 * @since 0.11.0
-	 * @since 3.0.0  Moved from the Sniff class to this dedicated Trait.
-	 *               Renamed from `$test_class_whitelist` to `$known_test_classes`.
+	 * @since 3.0.0  - Moved from the Sniff class to this dedicated Trait.
+	 *               - Renamed from `$test_class_whitelist` to `$known_test_classes`.
+	 *               - Visibility changed from protected to private.
 	 *
 	 * @var string[]
 	 */
-	protected $known_test_classes = array(
+	private $known_test_classes = array(
 		// Base test cases.
 		'WP_UnitTestCase'                            => true,
 		'WP_UnitTestCase_Base'                       => true,
@@ -100,6 +101,60 @@ trait IsUnitTestTrait {
 		'TestCase'                                   => true,
 	);
 
+	/**
+	 * Cache of previously added custom test classes.
+	 *
+	 * Prevents having to do the same merges over and over again.
+	 *
+	 * @since 3.0.0
+	 *
+	 * @var string[]
+	 */
+	private $added_custom_test_classes = array();
+
+	/**
+	 * Combined list of WP/PHPUnit native and custom test classes.
+	 *
+	 * @since 3.0.0
+	 *
+	 * @var array<string, bool>
+	 */
+	private $all_test_classes = array();
+
+	/**
+	 * Retrieve a list of all registered test classes, both WP/PHPUnit native as well as custom.
+	 *
+	 * @since 3.0.0
+	 *
+	 * @var array<string, bool>
+	 */
+	protected function get_all_test_classes() {
+		if ( array() === $this->all_test_classes
+			|| $this->custom_test_classes !== $this->added_custom_test_classes
+		) {
+			/*
+			 * Show some tolerance for user input.
+			 * The custom test class names should be passed as FQN without a prefixing `\`.
+			 */
+			$custom_test_classes = array();
+			if ( ! empty( $this->custom_test_classes ) ) {
+				foreach ( $this->custom_test_classes as $v ) {
+					$custom_test_classes[] = ltrim( $v, '\\' );
+				}
+			}
+
+			$this->all_test_classes = RulesetPropertyHelper::merge_custom_array(
+				$custom_test_classes,
+				$this->known_test_classes
+			);
+
+			// Store the original value so the comparison can succeed.
+			$this->added_custom_test_classes = $this->custom_test_classes;
+		}
+
+		return $this->all_test_classes;
+	}
+
 	/**
 	 * Check if a class token is part of a unit test suite.
 	 *
@@ -127,18 +182,7 @@ protected function is_test_class( File $phpcsFile, $stackPtr ) {
 		}
 
 		// Add any potentially extra custom test classes to the known test classes list.
-		$known_test_classes = RulesetPropertyHelper::merge_custom_array(
-			$this->custom_test_classes,
-			$this->known_test_classes
-		);
-
-		/*
-		 * Show some tolerance for user input.
-		 * The custom test class names should be passed as FQN without a prefixing `\`.
-		 */
-		foreach ( $known_test_classes as $k => $v ) {
-			$known_test_classes[ $k ] = ltrim( $v, '\\' );
-		}
+		$known_test_classes = $this->get_all_test_classes();
 
 		// Is the class/trait one of the known test classes ?
 		$namespace = Namespaces::determineNamespace( $phpcsFile, $stackPtr );
diff --git a/WordPress/Tests/Files/FileNameUnitTest.php b/WordPress/Tests/Files/FileNameUnitTest.php
index 13909316cf..0e8c48579f 100644
--- a/WordPress/Tests/Files/FileNameUnitTest.php
+++ b/WordPress/Tests/Files/FileNameUnitTest.php
@@ -75,7 +75,11 @@ final class FileNameUnitTest extends AbstractSniffUnitTest {
 		'test-sample-phpunit.inc'                    => 0,
 		'test-sample-phpunit6.inc'                   => 0,
 		'test-sample-wpunit.inc'                     => 0,
-		'test-sample-custom-unit.inc'                => 0,
+		'test-sample-custom-unit.1.inc'              => 0,
+		'test-sample-custom-unit.2.inc'              => 0,
+		'test-sample-custom-unit.3.inc'              => 0,
+		'test-sample-custom-unit.4.inc'              => 0,
+		'test-sample-custom-unit.5.inc'              => 1, // Namespaced vs non-namespaced.
 		'test-sample-namespaced-declaration.1.inc'   => 0,
 		'test-sample-namespaced-declaration.2.inc'   => 1, // Namespaced vs non-namespaced.
 		'test-sample-namespaced-declaration.3.inc'   => 1, // Wrong namespace.
diff --git a/WordPress/Tests/Files/FileNameUnitTests/TestFiles/test-sample-custom-unit.inc b/WordPress/Tests/Files/FileNameUnitTests/TestFiles/test-sample-custom-unit.1.inc
similarity index 100%
rename from WordPress/Tests/Files/FileNameUnitTests/TestFiles/test-sample-custom-unit.inc
rename to WordPress/Tests/Files/FileNameUnitTests/TestFiles/test-sample-custom-unit.1.inc
diff --git a/WordPress/Tests/Files/FileNameUnitTests/TestFiles/test-sample-custom-unit.2.inc b/WordPress/Tests/Files/FileNameUnitTests/TestFiles/test-sample-custom-unit.2.inc
new file mode 100644
index 0000000000..41170aa93d
--- /dev/null
+++ b/WordPress/Tests/Files/FileNameUnitTests/TestFiles/test-sample-custom-unit.2.inc
@@ -0,0 +1,6 @@
+<!-- Annotation has to be on the second line as errors are thrown on line 1 and errors on annotation lines are ignored. -->
+phpcs:set WordPress.Files.FileName custom_test_classes[] \My_TestClass
+<?php
+
+class TestSample extends My_TestClass {}
+/* phpcs:set WordPress.Files.FileName custom_test_classes[] */
diff --git a/WordPress/Tests/Files/FileNameUnitTests/TestFiles/test-sample-custom-unit.3.inc b/WordPress/Tests/Files/FileNameUnitTests/TestFiles/test-sample-custom-unit.3.inc
new file mode 100644
index 0000000000..ff77ee2774
--- /dev/null
+++ b/WordPress/Tests/Files/FileNameUnitTests/TestFiles/test-sample-custom-unit.3.inc
@@ -0,0 +1,8 @@
+<!-- Annotation has to be on the second line as errors are thrown on line 1 and errors on annotation lines are ignored. -->
+phpcs:set WordPress.Files.FileName custom_test_classes[] \Plugin\Tests\My_TestClass
+<?php
+
+namespace Plugin\Tests;
+
+class TestSample extends My_TestClass {}
+/* phpcs:set WordPress.Files.FileName custom_test_classes[] */
diff --git a/WordPress/Tests/Files/FileNameUnitTests/TestFiles/test-sample-custom-unit.4.inc b/WordPress/Tests/Files/FileNameUnitTests/TestFiles/test-sample-custom-unit.4.inc
new file mode 100644
index 0000000000..3df9b2c54a
--- /dev/null
+++ b/WordPress/Tests/Files/FileNameUnitTests/TestFiles/test-sample-custom-unit.4.inc
@@ -0,0 +1,6 @@
+<!-- Annotation has to be on the second line as errors are thrown on line 1 and errors on annotation lines are ignored. -->
+phpcs:set WordPress.Files.FileName custom_test_classes[] \My_TestClass
+<?php
+
+class TestSample extends \My_TestClass {}
+/* phpcs:set WordPress.Files.FileName custom_test_classes[] */
diff --git a/WordPress/Tests/Files/FileNameUnitTests/TestFiles/test-sample-custom-unit.5.inc b/WordPress/Tests/Files/FileNameUnitTests/TestFiles/test-sample-custom-unit.5.inc
new file mode 100644
index 0000000000..392a1546db
--- /dev/null
+++ b/WordPress/Tests/Files/FileNameUnitTests/TestFiles/test-sample-custom-unit.5.inc
@@ -0,0 +1,8 @@
+<!-- Annotation has to be on the second line as errors are thrown on line 1 and errors on annotation lines are ignored. -->
+phpcs:set WordPress.Files.FileName custom_test_classes[] \My_TestClass
+<?php
+
+namespace Plugin\Tests;
+
+class TestSample extends My_TestClass {}
+/* phpcs:set WordPress.Files.FileName custom_test_classes[] */