diff --git a/lib/client-assets.php b/lib/client-assets.php
index 2302345beaa8d..3030459fbfce8 100644
--- a/lib/client-assets.php
+++ b/lib/client-assets.php
@@ -840,6 +840,67 @@ function gutenberg_get_available_image_sizes() {
 	return $all_sizes;
 }
 
+/**
+ * Extends block editor settings to include Gutenberg's `editor-styles.css` as
+ * taking precedent those styles shipped with core.
+ *
+ * @param array $settings Default editor settings.
+ *
+ * @return array Filtered editor settings.
+ */
+function gutenberg_extend_block_editor_styles( $settings ) {
+	$editor_styles_file = gutenberg_dir_path() . 'build/editor/editor-styles.css';
+
+	/*
+	 * If, for whatever reason, the built editor styles do not exist, avoid
+	 * override and fall back to the default.
+	 */
+	if ( ! file_exists( $editor_styles_file ) ) {
+		return $settings;
+	}
+
+	if ( empty( $settings['styles'] ) ) {
+		$settings['styles'] = array();
+	} else {
+		/*
+		 * The styles setting is an array of CSS strings, so there is no direct
+		 * way to find the default styles. To maximize stability, load (again)
+		 * the default styles from disk and find its place in the array.
+		 *
+		 * See: https://github.com/WordPress/wordpress-develop/blob/5.0.3/src/wp-admin/edit-form-blocks.php#L168-L175
+		 */
+
+		$default_styles = file_get_contents(
+			ABSPATH . WPINC . '/css/dist/editor/editor-styles.css'
+		);
+
+		/*
+		 * Iterate backwards from the end of the array since the preferred
+		 * insertion point in case not found is prepended as first entry.
+		 */
+		for ( $i = count( $settings['styles'] ) - 1; $i >= 0; $i-- ) {
+			if ( isset( $settings['styles'][ $i ]['css'] ) &&
+					$default_styles === $settings['styles'][ $i ]['css'] ) {
+				break;
+			}
+		}
+	}
+
+	$editor_styles = array(
+		'css' => file_get_contents( $editor_styles_file ),
+	);
+
+	// Substitute default styles if found. Otherwise, prepend to setting array.
+	if ( isset( $i ) && $i >= 0 ) {
+		$settings['styles'][ $i ] = $editor_styles;
+	} else {
+		array_unshift( $settings['styles'], $editor_styles );
+	}
+
+	return $settings;
+}
+add_filter( 'block_editor_settings', 'gutenberg_extend_block_editor_styles' );
+
 /**
  * Scripts & Styles.
  *
@@ -1002,7 +1063,7 @@ function gutenberg_editor_scripts_and_styles( $hook ) {
 	$styles = array(
 		array(
 			'css' => file_get_contents(
-				gutenberg_dir_path() . 'build/editor/editor-styles.css'
+				ABSPATH . WPINC . '/css/dist/editor/editor-styles.css'
 			),
 		),
 	);
diff --git a/phpunit/class-extend-styles-test.php b/phpunit/class-extend-styles-test.php
new file mode 100644
index 0000000000000..6ae12ada22797
--- /dev/null
+++ b/phpunit/class-extend-styles-test.php
@@ -0,0 +1,185 @@
+<?php
+/**
+ * Test `gutenberg_extend_block_editor_styles`.
+ *
+ * @package Gutenberg
+ */
+
+class Extend_Styles_Test extends WP_UnitTestCase {
+
+	/**
+	 * Path of the original `editor-styles.css` file.
+	 *
+	 * @var string|null
+	 */
+	protected $orignal_file = null;
+
+	/**
+	 * Contents of the `editor-styles.css` file.
+	 *
+	 * @var string
+	 */
+	protected $style_contents = null;
+
+	public function wpTearDown() {
+		parent::wpTearDown();
+
+		$this->restore_editor_styles();
+	}
+
+	/**
+	 * Restores the existence of `editor-styles.css` to its original state.
+	 */
+	protected function restore_editor_styles() {
+		$path = gutenberg_dir_path() . 'build/editor/editor-styles.css';
+
+		if ( $this->original_file ) {
+			if ( $this->original_file !== $path ) {
+				rename( $this->original_file, $path );
+			}
+		} elseif ( file_exists( $path ) ) {
+			unlink( $path );
+		}
+
+		$this->style_contents = null;
+		$this->original_file  = null;
+	}
+
+	/**
+	 * Guarantees that an `editor-styles.css` file exists, if and only if it
+	 * should exist. Assigns `style_contents` according to the contents of the
+	 * file if it should exist. Renames the existing file temporarily if it
+	 * exists but should not.
+	 *
+	 * @param bool $should_exist Whether the editor styles file should exist.
+	 */
+	protected function ensure_editor_styles( $should_exist = true ) {
+		$path = gutenberg_dir_path() . 'build/editor/editor-styles.css';
+
+		if ( file_exists( $path ) ) {
+			if ( $should_exist ) {
+				$this->style_contents = file_get_contents( $path );
+				$this->original_file  = $path;
+			} else {
+				rename( $path, $path . '.bak' );
+				$this->original_file = $path . '.bak';
+			}
+		} elseif ( $should_exist ) {
+			$this->style_contents = '';
+			file_put_contents( $path, $this->style_contents );
+			$this->original_file = null;
+		}
+	}
+
+	/**
+	 * Tests a non-existent build `styles`.
+	 */
+	function test_without_built_styles() {
+		$this->ensure_editor_styles( false );
+
+		$settings = array(
+			'styles' => array(
+				array( 'css' => 'original' ),
+				array( 'css' => 'someother' ),
+			),
+		);
+
+		$result = gutenberg_extend_block_editor_styles( $settings );
+
+		$this->assertEquals( $settings, $result );
+	}
+
+	/**
+	 * Tests an unset `styles` setting.
+	 */
+	function test_unset_editor_settings_style() {
+		$this->ensure_editor_styles();
+
+		$settings = array();
+
+		$settings = gutenberg_extend_block_editor_styles( $settings );
+
+		$this->assertEquals(
+			array( array( 'css' => $this->style_contents ) ),
+			$settings['styles']
+		);
+	}
+
+	/**
+	 * Tests replacing the default styles.
+	 */
+	function test_replace_default_editor_styles() {
+		$this->ensure_editor_styles();
+		$default_styles = file_get_contents(
+			ABSPATH . WPINC . '/css/dist/editor/editor-styles.css'
+		);
+
+		$settings = array(
+			'styles' => array(
+				array( 'css' => $default_styles ),
+				array( 'css' => 'someother' ),
+			),
+		);
+
+		$settings = gutenberg_extend_block_editor_styles( $settings );
+
+		$this->assertEquals(
+			array(
+				array( 'css' => $this->style_contents ),
+				array( 'css' => 'someother' ),
+			),
+			$settings['styles']
+		);
+	}
+
+	/**
+	 * Tests replacing the rearranged default styles.
+	 */
+	function test_replace_rearranged_default_editor_styles() {
+		$this->ensure_editor_styles();
+		$default_styles = file_get_contents(
+			ABSPATH . WPINC . '/css/dist/editor/editor-styles.css'
+		);
+
+		$settings = array(
+			'styles' => array(
+				array( 'css' => 'someother' ),
+				array( 'css' => $default_styles ),
+			),
+		);
+
+		$settings = gutenberg_extend_block_editor_styles( $settings );
+
+		$this->assertEquals(
+			array(
+				array( 'css' => 'someother' ),
+				array( 'css' => $this->style_contents ),
+			),
+			$settings['styles']
+		);
+	}
+
+	/**
+	 * Tests when the default styles aren't in the styles setting.
+	 */
+	function test_without_default_editor_styles() {
+		$this->ensure_editor_styles();
+
+		$settings = array(
+			'styles' => array(
+				array( 'css' => 'someother' ),
+			),
+		);
+
+		$settings = gutenberg_extend_block_editor_styles( $settings );
+
+		$this->assertEquals(
+			array(
+				array( 'css' => $this->style_contents ),
+				array( 'css' => 'someother' ),
+			),
+			$settings['styles']
+		);
+	}
+
+}