diff --git src/wp-includes/media.php src/wp-includes/media.php
index df96a17..086d50e 100644
--- src/wp-includes/media.php
+++ src/wp-includes/media.php
@@ -1142,7 +1142,7 @@ function wp_calculate_image_srcset( $size_array, $image_src, $image_meta, $attac
 	 * @param array  $size_array    Array of width and height values in pixels (in that order).
 	 * @param string $image_src     The 'src' of the image.
 	 * @param array  $image_meta    The image meta data as returned by 'wp_get_attachment_metadata()'.
- 	 * @param int    $attachment_id Image attachment ID or 0.
+	 * @param int    $attachment_id Image attachment ID or 0.
 	 */
 	$sources = apply_filters( 'wp_calculate_image_srcset', $sources, $size_array, $image_src, $image_meta, $attachment_id );
 
@@ -1154,7 +1154,7 @@ function wp_calculate_image_srcset( $size_array, $image_src, $image_meta, $attac
 	$srcset = '';
 
 	foreach ( $sources as $source ) {
-		$srcset .= $source['url'] . ' ' . $source['value'] . $source['descriptor'] . ', ';
+		$srcset .= str_replace( ' ', '%20', $source['url'] ) . ' ' . $source['value'] . $source['descriptor'] . ', ';
 	}
 
 	return rtrim( $srcset, ', ' );
diff --git tests/phpunit/tests/media.php tests/phpunit/tests/media.php
index 7bd9f49..5d8449f 100644
--- tests/phpunit/tests/media.php
+++ tests/phpunit/tests/media.php
@@ -1322,6 +1322,50 @@ EOF;
 	}
 
 	/**
+	 * @ticket 36549
+	 * @ticket 33641
+	 */
+	function test_wp_calculate_image_srcset_with_spaces_in_filenames() {
+		// Mock data for this test.
+		$image_src = 'http://' . WP_TESTS_DOMAIN . '/wp-content/uploads/2015/12/test image-300x150.png';
+		$image_meta = array(
+			'width' => 2000,
+			'height' => 1000,
+			'file' => '2015/12/test image.png',
+			'sizes' => array(
+				'thumbnail' => array(
+					'file' => 'test image-150x150.png',
+					'width' => 150,
+					'height' => 150,
+					'mime-type' => 'image/png',
+				),
+				'medium' => array(
+					'file' => 'test image-300x150.png',
+					'width' => 300,
+					'height' => 150,
+					'mime-type' => 'image/png',
+				),
+				'medium_large' => array(
+					'file' => 'test image-768x384.png',
+					'width' => 768,
+					'height' => 384,
+					'mime-type' => 'image/png',
+				),
+				'large' => array(
+					'file' => 'test image-1024x512.png',
+					'width' => 1024,
+					'height' => 512,
+					'mime-type' => 'image/png',
+				),
+			),
+		);
+
+		$expected_srcset = 'http://' . WP_TESTS_DOMAIN . '/wp-content/uploads/2015/12/test%20image-300x150.png 300w, http://' . WP_TESTS_DOMAIN . '/wp-content/uploads/2015/12/test%20image-768x384.png 768w, http://' . WP_TESTS_DOMAIN . '/wp-content/uploads/2015/12/test%20image-1024x512.png 1024w';
+
+		$this->assertSame( $expected_srcset, wp_calculate_image_srcset( array( 300, 150 ), $image_src, $image_meta ) );
+	}
+
+	/**
 	 * @ticket 33641
 	 */
 	function test_wp_get_attachment_image_srcset() {