diff --git a/tests/phpunit/tests/media/wpImageTagAddDecodingAttr.php b/tests/phpunit/tests/media/wpImageTagAddDecodingAttr.php
new file mode 100644
index 0000000000..76fa203b7e
--- /dev/null
+++ b/tests/phpunit/tests/media/wpImageTagAddDecodingAttr.php
@@ -0,0 +1,164 @@
+<?php
+
+/**
+ * Tests the `wp_img_tag_add_decoding_attr()` function.
+ *
+ * @group media
+ * @covers ::wp_img_tag_add_decoding_attr
+ */
+class Tests_Media_Wp_Img_Tag_Add_Decoding_Attr extends WP_UnitTestCase {
+	/**
+	 * Tests that the `wp_img_tag_add_decoding_attr()` function should add
+	 * the 'decoding' attribute.
+	 *
+	 * @ticket 53232
+	 *
+	 * @dataProvider data_should_add_decoding_attr
+	 *
+	 * @param string $image    The HTML `img` tag where the attribute should be added.
+	 * @param string $context  Additional context to pass to the filters.
+	 * @param string $decoding The value for the 'decoding' attribute. 'no value' for default.
+	 * @param string $expected The expected `img` tag.
+	 */
+	public function test_should_add_decoding_attr( $image, $context, $decoding, $expected ) {
+		// Falsey values are allowed in the filter, cannot use `null` or `false` here.
+		if ( 'no value' !== $decoding ) {
+			add_filter(
+				'wp_img_tag_add_decoding_attr',
+				static function( $value ) use ( $decoding ) {
+					return $decoding;
+				}
+			);
+		}
+
+		$this->assertSame( $expected, wp_img_tag_add_decoding_attr( $image, $context ) );
+	}
+
+	/**
+	 * Data provider.
+	 *
+	 * @return array
+	 */
+	public function data_should_add_decoding_attr() {
+		return array(
+			'default' => array(
+				'image'    => '<img src="my-image.png">',
+				'context'  => '',
+				'decoding' => 'no value',
+				'expected' => '<img decoding="async" src="my-image.png">',
+			),
+			'async'   => array(
+				'image'    => '<img src="my-image.png">',
+				'context'  => '',
+				'decoding' => 'async',
+				'expected' => '<img decoding="async" src="my-image.png">',
+			),
+			'sync'    => array(
+				'image'    => '<img src="my-image.png">',
+				'context'  => '',
+				'decoding' => 'sync',
+				'expected' => '<img decoding="sync" src="my-image.png">',
+			),
+			'auto'    => array(
+				'image'    => '<img src="my-image.png">',
+				'context'  => '',
+				'decoding' => 'auto',
+				'expected' => '<img decoding="auto" src="my-image.png">',
+			),
+		);
+	}
+
+	/**
+	 * Tests that the `wp_img_tag_add_decoding_attr()` function should not add
+	 * the 'decoding' attribute.
+	 *
+	 * @ticket 53232
+	 *
+	 * @dataProvider data_should_not_add_decoding_attr
+	 *
+	 * @param string $image    The HTML `img` tag where the attribute should be added.
+	 * @param string $context  Additional context to pass to the filters.
+	 * @param mixed  $decoding The value for the 'decoding' attribute. 'no value' for default.
+	 * @param string $expected The expected `img` tag.
+	 */
+	public function test_should_not_add_decoding_attr( $image, $context, $decoding, $expected ) {
+		// Falsey values are allowed in the filter, cannot use `null` or `false` here.
+		if ( 'no value' !== $decoding ) {
+			add_filter(
+				'wp_img_tag_add_decoding_attr',
+				static function( $value ) use ( $decoding ) {
+					return $decoding;
+				}
+			);
+		}
+
+		$this->assertSame( $expected, wp_img_tag_add_decoding_attr( $image, $context, $expected ) );
+	}
+
+	/**
+	 * Data provider.
+	 *
+	 * @return array
+	 */
+	public function data_should_not_add_decoding_attr() {
+		return array(
+			// Unhappy paths.
+			'lazy (unaccepted value)' => array(
+				'image'    => '<img src="my-image.png">',
+				'context'  => '',
+				'decoding' => 'lazy',
+				'expected' => '<img src="my-image.png">',
+			),
+			'a non-string value'      => array(
+				'image'    => '<img src="my-image.png">',
+				'context'  => '',
+				'decoding' => array( 'sync' ),
+				'expected' => '<img src="my-image.png">',
+			),
+
+			// Falsey values.
+			'false'                   => array(
+				'image'    => '<img src="my-image.png">',
+				'context'  => '',
+				'decoding' => false,
+				'expected' => '<img src="my-image.png">',
+			),
+			'null'                    => array(
+				'image'    => '<img src="my-image.png">',
+				'context'  => '',
+				'decoding' => null,
+				'expected' => '<img src="my-image.png">',
+			),
+			'empty string'            => array(
+				'image'    => '<img src="my-image.png">',
+				'context'  => '',
+				'decoding' => '',
+				'expected' => '<img src="my-image.png">',
+			),
+			'empty array'             => array(
+				'image'    => '<img src="my-image.png">',
+				'context'  => '',
+				'decoding' => array(),
+				'expected' => '<img src="my-image.png">',
+			),
+			'0 int'                   => array(
+				'image'    => '<img src="my-image.png">',
+				'context'  => '',
+				'decoding' => 0,
+				'expected' => '<img src="my-image.png">',
+			),
+			'0 string'                => array(
+				'image'    => '<img src="my-image.png">',
+				'context'  => '',
+				'decoding' => '0',
+				'expected' => '<img src="my-image.png">',
+			),
+			'0.0 float'               => array(
+				'image'    => '<img src="my-image.png">',
+				'context'  => '',
+				'decoding' => 0.0,
+				'expected' => '<img src="my-image.png">',
+			),
+		);
+	}
+}