diff --git a/tests/phpunit/tests/functions/_defaultWpDieHandler.php b/tests/phpunit/tests/functions/_defaultWpDieHandler.php
new file mode 100644
index 0000000000000..19f36de83da40
--- /dev/null
+++ b/tests/phpunit/tests/functions/_defaultWpDieHandler.php
@@ -0,0 +1,78 @@
+<?php
+
+/**
+ * Tests the _default_wp_die_handler() function.
+ *
+ * @group functions
+ *
+ * @covers ::_default_wp_die_handler
+ */
+class Tests_Functions_DefaultWpDieHandler extends WP_UnitTestCase {
+
+	/**
+	 * Tests the default HTML output of the handler.
+	 *
+	 * @ticket 65655
+	 */
+	public function test_default_wp_die_handler_output() {
+		ob_start();
+		try {
+			_default_wp_die_handler( 'Error Message', 'Error Title', array( 'exit' => false ) );
+		} catch ( WPDieException $e ) {
+		}
+		$output = ob_get_clean();
+
+		$this->assertStringContainsString( '<div class="wp-die-message">Error Message</div>', $output );
+		$this->assertStringContainsString( '<title>Error Title</title>', $output );
+	}
+
+	/**
+	 * Tests handling of additional errors.
+	 *
+	 * @ticket 65655
+	 */
+	public function test_default_wp_die_handler_additional_errors() {
+		$args = array(
+			'exit'              => false,
+			'additional_errors' => array(
+				array( 'message' => 'Extra Error 1' ),
+				array( 'message' => 'Extra Error 2' ),
+			),
+		);
+
+		ob_start();
+		try {
+			_default_wp_die_handler( 'Main Error', '', $args );
+		} catch ( WPDieException $e ) {
+		}
+		$output = ob_get_clean();
+
+		$this->assertStringContainsString( '<li>Main Error</li>', $output );
+		$this->assertStringContainsString( '<li>Extra Error 1</li>', $output );
+		$this->assertStringContainsString( '<li>Extra Error 2</li>', $output );
+	}
+
+	/**
+	 * Tests handling of a link.
+	 *
+	 * @ticket 65655
+	 */
+	public function test_default_wp_die_handler_link() {
+		$args = array(
+			'exit'      => false,
+			'link_url'  => 'https://example.com',
+			'link_text' => 'Go to Example',
+		);
+
+		ob_start();
+		try {
+			_default_wp_die_handler( 'Error', '', $args );
+		} catch ( WPDieException $e ) {
+		}
+		$output = ob_get_clean();
+
+		// The default handler uses single quotes for attributes in some cases.
+		$this->assertStringContainsString( "href='https://example.com'", $output );
+		$this->assertStringContainsString( '>Go to Example</a>', $output );
+	}
+}
diff --git a/tests/phpunit/tests/functions/wpDie.php b/tests/phpunit/tests/functions/wpDie.php
new file mode 100644
index 0000000000000..f53abdd1b4c4c
--- /dev/null
+++ b/tests/phpunit/tests/functions/wpDie.php
@@ -0,0 +1,83 @@
+<?php
+
+/**
+ * Tests the wp_die() function.
+ *
+ * @group functions
+ *
+ * @covers ::wp_die
+ */
+class Tests_Functions_WpDie extends WP_UnitTestCase {
+
+	/**
+	 * Tests that wp_die() calls the expected handler.
+	 *
+	 * @ticket 65655
+	 */
+	public function test_wp_die_handler_selection() {
+		$filter = new MockAction();
+		add_filter( 'wp_die_handler', array( $filter, 'filter' ) );
+
+		try {
+			wp_die( 'Message', 'Title', array( 'exit' => false ) );
+		} catch ( WPDieException $e ) {
+			// The default test handler throws this.
+		}
+
+		$this->assertSame( 1, $filter->get_call_count(), 'The wp_die_handler filter should have been called once.' );
+	}
+
+	/**
+	 * Tests that wp_die() respects the Ajax handler filter.
+	 *
+	 * @ticket 65655
+	 *
+	 * @runInSeparateProcess
+	 * @preserveGlobalState disabled
+	 */
+	public function test_wp_die_ajax_handler_selection() {
+		if ( ! defined( 'DOING_AJAX' ) ) {
+			define( 'DOING_AJAX', true );
+		}
+
+		$filter = new MockAction();
+		add_filter( 'wp_die_ajax_handler', array( $filter, 'filter' ) );
+
+		ob_start();
+		try {
+			wp_die( 'Ajax Message', 'Ajax Title', array( 'exit' => false ) );
+		} catch ( WPDieException $e ) {
+		}
+		ob_end_clean();
+
+		$this->assertSame( 1, $filter->get_call_count(), 'The wp_die_ajax_handler filter should have been called once.' );
+	}
+
+	/**
+	 * Tests that wp_die() handles WP_Error objects.
+	 *
+	 * @ticket 65655
+	 */
+	public function test_wp_die_with_wp_error() {
+		$error = new WP_Error( 'test_error', 'Test Error Message' );
+
+		// Use a simple variable to capture the first argument.
+		$captured_message = null;
+		add_filter(
+			'wp_die_handler',
+			function ( $callback ) use ( &$captured_message ) {
+				return function ( $message ) use ( &$captured_message ) {
+					$captured_message = $message;
+					throw new WPDieException( 'Intercepted' );
+				};
+			}
+		);
+
+		try {
+			wp_die( $error, '', array( 'exit' => false ) );
+		} catch ( WPDieException $e ) {
+		}
+
+		$this->assertSame( $error, $captured_message, 'The error object should be passed as the first argument to the handler.' );
+	}
+}