diff --git a/tests/phpunit/tests/functions/statusHeader.php b/tests/phpunit/tests/functions/statusHeader.php
new file mode 100644
index 0000000000000..e3ec9c9ff515f
--- /dev/null
+++ b/tests/phpunit/tests/functions/statusHeader.php
@@ -0,0 +1,134 @@
+<?php
+
+/**
+ * Tests the status_header() function.
+ *
+ * @group functions
+ *
+ * @covers ::status_header
+ */
+class Tests_Functions_StatusHeader extends WP_UnitTestCase {
+
+	/**
+	 * Tests that status_header() correctly filters the header.
+	 *
+	 * Since we cannot easily test the actual header() call in PHPUnit,
+	 * we rely on the 'status_header' filter to verify the generated string.
+	 *
+	 * @ticket 65650
+	 *
+	 * @dataProvider data_status_header
+	 *
+	 * @param int    $code           HTTP status code.
+	 * @param string $description    Custom description.
+	 * @param string $expected_regex Regex to match the generated status header.
+	 */
+	public function test_status_header_filter( $code, $description, $expected_regex ) {
+		$filter = new MockAction();
+		add_filter( 'status_header', array( $filter, 'filter' ), 10, 4 );
+
+		status_header( $code, $description );
+
+		$args = $filter->get_args();
+		$this->assertNotEmpty( $args, 'The status_header filter was not called.' );
+
+		$actual_header = $args[0][0];
+		$this->assertMatchesRegularExpression( $expected_regex, $actual_header );
+		$this->assertSame( (int) $code, $args[0][1], 'The status code passed to the filter does not match.' );
+
+		if ( $description ) {
+			$this->assertSame( $description, $args[0][2], 'The description passed to the filter does not match.' );
+		} else {
+			$this->assertSame( get_status_header_desc( $code ), $args[0][2], 'The default description passed to the filter does not match.' );
+		}
+	}
+
+	/**
+	 * Data provider for test_status_header_filter().
+	 *
+	 * @return array<string, array{
+	 *     code:           int,
+	 *     description:    string,
+	 *     expected_regex: string,
+	 * }>
+	 */
+	public function data_status_header(): array {
+		return array(
+			'200 OK'                => array(
+				200,
+				'',
+				'/^HTTP\/1\.[012] 200 OK$/',
+			),
+			'404 Not Found'         => array(
+				404,
+				'',
+				'/^HTTP\/1\.[012] 404 Not Found$/',
+			),
+			'Custom description'    => array(
+				200,
+				'Everything is Fine',
+				'/^HTTP\/1\.[012] 200 Everything is Fine$/',
+			),
+			'301 Moved Permanently' => array(
+				301,
+				'',
+				'/^HTTP\/1\.[012] 301 Moved Permanently$/',
+			),
+		);
+	}
+
+	/**
+	 * Tests that status_header() returns early for unknown status codes without a description.
+	 *
+	 * @ticket 65650
+	 */
+	public function test_status_header_unknown_code_returns_early() {
+		$filter = new MockAction();
+		add_filter( 'status_header', array( $filter, 'filter' ), 10, 4 );
+
+		status_header( 999 );
+
+		$this->assertEmpty( $filter->get_args(), 'The status_header filter should not have been called for an unknown status code without description.' );
+	}
+
+	/**
+	 * Tests that status_header() works with unknown status codes if a description is provided.
+	 *
+	 * @ticket 65650
+	 */
+	public function test_status_header_unknown_code_with_description() {
+		$filter = new MockAction();
+		add_filter( 'status_header', array( $filter, 'filter' ), 10, 4 );
+
+		status_header( 999, 'Custom Status' );
+
+		$args = $filter->get_args();
+		$this->assertNotEmpty( $args, 'The status_header filter should have been called when a description is provided.' );
+		$this->assertMatchesRegularExpression( '/^HTTP\/1\.[012] 999 Custom Status$/', $args[0][0] );
+	}
+
+	/**
+	 * Tests that status_header() respects the server protocol.
+	 *
+	 * @ticket 65650
+	 */
+	public function test_status_header_respects_protocol() {
+		$old_protocol               = $_SERVER['SERVER_PROTOCOL'] ?? null;
+		$_SERVER['SERVER_PROTOCOL'] = 'HTTP/1.1';
+
+		$filter = new MockAction();
+		add_filter( 'status_header', array( $filter, 'filter' ), 10, 4 );
+
+		status_header( 200 );
+
+		$args = $filter->get_args();
+		$this->assertStringStartsWith( 'HTTP/1.1 200 OK', $args[0][0] );
+
+		// Reset protocol.
+		if ( null === $old_protocol ) {
+			unset( $_SERVER['SERVER_PROTOCOL'] );
+		} else {
+			$_SERVER['SERVER_PROTOCOL'] = $old_protocol;
+		}
+	}
+}