diff --git a/tests/phpunit/tests/media/sideload.php b/tests/phpunit/tests/media/sideload.php
new file mode 100644
index 000000000..085e5b042
--- /dev/null
+++ b/tests/phpunit/tests/media/sideload.php
@@ -0,0 +1,106 @@
+<?php
+
+/**
+ * @group media
+ * @group sideload
+ */
+class Tests_Media_Sideload extends WP_UnitTestCase {
+	private $attachment_id = 0;
+
+	public function setUp() {
+		parent::setUp();
+
+		add_filter( 'pre_http_request', array( $this, 'mock_sideload_request' ), 10, 3 );
+		add_filter( 'wp_update_attachment_metadata', array( $this, 'get_attachment_metadata' ), 10, 2 );
+	}
+
+	public function tearDown() {
+		parent::tearDown();
+
+		remove_filter( 'pre_http_request', array( $this, 'mock_sideload_request' ), 10, 3 );
+		remove_filter( 'wp_update_attachment_metadata', array( $this, 'get_attachment_metadata' ), 10, 2 );
+	}
+
+	/**
+	 * Catch the attachment ID as attachment meta is updated so that it
+	 * can be used to check against `media_sideload_image` output.
+	 *
+	 * @param array $data Attachment metadata.
+	 * @param int   $id   Attachment ID.
+	 * @return array Attachment metadata.
+	 */
+	public function get_attachment_metadata( $data, $id ) {
+		$this->attachment_id = $id;
+		return $data;
+	}
+
+	/**
+	 * Intercept image sideload requests and mock responses.
+	 *
+	 * @param mixed  $preempt Whether to preempt an HTTP request's return value. Default false.
+	 * @param mixed  $r       HTTP request arguments.
+	 * @param string $url     The request URL.
+	 * @return array Response data.
+	 */
+	public function mock_sideload_request( $preempt, $r, $url ) {
+		// If `$url` is set to false, continue the request to
+		// simulate failure using an improper URL.
+		if ( false == $url ) {
+			return $preempt;
+		}
+
+		// Copy the existing file to the expected temporary location to
+		// simulate a successful upload.
+		copy( DIR_TESTDATA . '/images/test-image.jpg', $r['filename'] );
+		return array(
+			'response' => array(
+				'code' => 200,
+			),
+		);
+	}
+
+	/**
+	 * Tests basic functionality and error handling of `media_sideload_image()`.
+	 *
+	 * @ticket 19629
+	 */
+	function test_wp_media_sideload_image() {
+		$source = 'https://example.com/test-image.jpg';
+		$alt = 'alt text';
+
+		// Test default return value, which should return the same as html.
+		$result = media_sideload_image( $source, 0, $alt );
+		$dest = wp_get_attachment_url( $this->attachment_id );
+
+		$expected = "<img src='$dest' alt='$alt' />";
+		$this->assertEquals( $expected, $result );
+		wp_delete_file( get_attached_file( $this->attachment_id ) );
+
+		// Test 'html' return value
+		$result = media_sideload_image( $source, 0, $alt, 'html' );
+		$dest = wp_get_attachment_url( $this->attachment_id );
+
+		$expected = "<img src='$dest' alt='$alt' />";
+		$this->assertEquals( $expected, $result );
+		wp_delete_file( get_attached_file( $this->attachment_id ) );
+
+		// Test 'id' return value
+		$result = media_sideload_image( $source, 0, $alt, 'id' );
+		$this->assertEquals( $this->attachment_id, $result);
+		wp_delete_file( get_attached_file( $this->attachment_id ) );
+
+		// Test 'src' return value
+		$result = media_sideload_image( $source, 0, $alt, 'src' );
+		$expected = wp_get_attachment_url( $this->attachment_id );
+		$this->assertEquals( $expected, $result);
+		wp_delete_file( get_attached_file( $this->attachment_id ) );
+
+		// Test WP_Error on invalid URL.
+		$result = media_sideload_image( false, 0, $alt, 'id' );
+		$this->assertInstanceOf( 'WP_Error', $result);
+
+		// Test WP_Error if file can't be sanitized to image.
+		$result = media_sideload_image( 'invalid-image.html', 0, $alt, 'id' );
+		$this->assertInstanceOf( 'WP_Error', $result);
+	}
+}