diff --git a/wp-admin/includes/schema.php b/wp-admin/includes/schema.php
index 05e82cee21..4f49dc21e1 100644
--- a/wp-admin/includes/schema.php
+++ b/wp-admin/includes/schema.php
@@ -368,11 +368,18 @@ function populate_options() {
 		$uploads_use_yearmonth_folders = 1;
 	}
 
-	$template = WP_DEFAULT_THEME;
-	// If default theme is a child theme, we need to get its template
-	$theme = wp_get_theme( $template );
-	if ( ! $theme->errors() )
-		$template = $theme->get_template();
+	// If WP_DEFAULT_THEME doesn't exist, fall back to the latest core default theme.
+	$stylesheet = $template = WP_DEFAULT_THEME;
+	$theme = wp_get_theme( WP_DEFAULT_THEME );
+	if ( ! $theme->exists() ) {
+		$theme = WP_Theme::get_core_default_theme();
+	}
+
+	// If we can't find a core default theme, WP_DEFAULT_THEME is the best we can do.
+	if ( $theme ) {
+		$stylesheet = $theme->get_stylesheet();
+		$template   = $theme->get_template();
+	}
 
 	$timezone_string = '';
 	$gmt_offset = 0;
@@ -433,7 +440,7 @@ function populate_options() {
 	'default_email_category' => 1,
 	'recently_edited' => '',
 	'template' => $template,
-	'stylesheet' => WP_DEFAULT_THEME,
+	'stylesheet' => $stylesheet,
 	'comment_whitelist' => 1,
 	'blacklist_keys' => '',
 	'comment_registration' => 0,
@@ -914,10 +921,21 @@ function populate_network( $network_id = 1, $domain = '', $email = '', $site_nam
 	$template = get_option( 'template' );
 	$stylesheet = get_option( 'stylesheet' );
 	$allowed_themes = array( $stylesheet => true );
-	if ( $template != $stylesheet )
+
+	if ( $template != $stylesheet ) {
 		$allowed_themes[ $template ] = true;
-	if ( WP_DEFAULT_THEME != $stylesheet && WP_DEFAULT_THEME != $template )
+	}
+
+	if ( WP_DEFAULT_THEME != $stylesheet && WP_DEFAULT_THEME != $template ) {
 		$allowed_themes[ WP_DEFAULT_THEME ] = true;
+	}
+
+	// If WP_DEFAULT_THEME doesn't exist, also whitelist the latest core default theme.
+	if ( ! wp_get_theme( WP_DEFAULT_THEME )->exists() ) {
+		if ( $core_default = WP_Theme::get_core_default_theme() ) {
+			$allowed_themes[ $core_default->get_stylesheet() ] = true;
+		}
+	}
 
 	if ( 1 == $network_id ) {
 		$wpdb->insert( $wpdb->site, array( 'domain' => $domain, 'path' => $path ) );
diff --git a/wp-admin/includes/update-core.php b/wp-admin/includes/update-core.php
index a6015e1516..9f17a086d9 100644
--- a/wp-admin/includes/update-core.php
+++ b/wp-admin/includes/update-core.php
@@ -715,6 +715,9 @@ $_old_files = array(
  * Directories should be noted by suffixing it with a trailing slash (/)
  *
  * @since 3.2.0
+ * @since 4.4.0 New themes are not automatically installed on upgrade.
+ *              This can still be explicitly asked for by defining
+ *              CORE_UPGRADE_SKIP_NEW_BUNDLED as false.
  * @global array $_new_bundled_files
  * @var array
  * @name $_new_bundled_files
@@ -732,6 +735,11 @@ $_new_bundled_files = array(
 	'themes/twentysixteen/'  => '4.4',
 );
 
+// If not explicitly defined as false, don't install new default themes.
+if ( ! defined( 'CORE_UPGRADE_SKIP_NEW_BUNDLED' ) || CORE_UPGRADE_SKIP_NEW_BUNDLED ) {
+	$_new_bundled_files = array( 'plugins/akismet/' => '2.0' );
+}
+
 /**
  * Upgrade the core of WordPress.
  *
diff --git a/wp-includes/class-wp-theme.php b/wp-includes/class-wp-theme.php
index e7e142d6fb..2128df230f 100644
--- a/wp-includes/class-wp-theme.php
+++ b/wp-includes/class-wp-theme.php
@@ -56,6 +56,7 @@ final class WP_Theme implements ArrayAccess {
 		'twentythirteen' => 'Twenty Thirteen',
 		'twentyfourteen' => 'Twenty Fourteen',
 		'twentyfifteen'  => 'Twenty Fifteen',
+		'twentysixteen'  => 'Twenty Sixteen',
 	);
 
 	/**
@@ -1150,6 +1151,23 @@ final class WP_Theme implements ArrayAccess {
 		return false;
 	}
 
+	/**
+	 * Determines the latest WordPress default theme that is installed.
+	 *
+	 * This hits the filesystem.
+	 *
+	 * @return WP_Theme|false Object, or false if no theme is installed, which would be bad.
+	 */
+	public static function get_core_default_theme() {
+		foreach ( array_reverse( self::$default_themes ) as $slug => $name ) {
+			$theme = wp_get_theme( $slug );
+			if ( $theme->exists() ) {
+				return $theme;
+			}
+		}
+		return false;
+	}
+
 	/**
 	 * Returns array of stylesheet names of themes allowed on the site or network.
 	 *
diff --git a/wp-includes/default-constants.php b/wp-includes/default-constants.php
index 12d6123023..774446e9e4 100644
--- a/wp-includes/default-constants.php
+++ b/wp-includes/default-constants.php
@@ -348,8 +348,10 @@ function wp_templating_constants() {
 	/**
 	 * Slug of the default theme for this install.
 	 * Used as the default theme when installing new sites.
-	 * Will be used as the fallback if the current theme doesn't exist.
+	 * It will be used as the fallback if the current theme doesn't exist.
+	 *
 	 * @since 3.0.0
+	 * @see WP_Theme::get_core_default_theme()
 	 */
 	if ( !defined('WP_DEFAULT_THEME') )
 		define( 'WP_DEFAULT_THEME', 'twentyfifteen' );
diff --git a/wp-includes/theme.php b/wp-includes/theme.php
index 395d42810a..47c9661778 100644
--- a/wp-includes/theme.php
+++ b/wp-includes/theme.php
@@ -753,8 +753,10 @@ function switch_theme( $stylesheet ) {
 /**
  * Checks that current theme files 'index.php' and 'style.css' exists.
  *
- * Does not check the default theme, which is the fallback and should always exist.
+ * Does not initially check the default theme, which is the fallback and should always exist.
+ * But if it doesn't exist, it'll fall back to the latest core default theme that does exist.
  * Will switch theme to the fallback theme if current theme does not validate.
+ *
  * You can use the 'validate_current_theme' filter to return false to
  * disable this functionality.
  *
@@ -774,22 +776,39 @@ function validate_current_theme() {
 	if ( wp_installing() || ! apply_filters( 'validate_current_theme', true ) )
 		return true;
 
-	if ( get_template() != WP_DEFAULT_THEME && !file_exists(get_template_directory() . '/index.php') ) {
+	if ( ! file_exists( get_template_directory() . '/index.php' ) ) {
+		// Invalid.
+	} elseif ( ! file_exists( get_template_directory() . '/style.css' ) ) {
+		// Invalid.
+	} elseif ( is_child_theme() && ! file_exists( get_stylesheet_directory() . '/style.css' ) ) {
+		// Invalid.
+	} else {
+		// Valid.
+		return true;
+	}
+
+	$default = wp_get_theme( WP_DEFAULT_THEME );
+	if ( $default->exists() ) {
 		switch_theme( WP_DEFAULT_THEME );
 		return false;
 	}
 
-	if ( get_stylesheet() != WP_DEFAULT_THEME && !file_exists(get_template_directory() . '/style.css') ) {
-		switch_theme( WP_DEFAULT_THEME );
-		return false;
+	/**
+	 * If we're in an invalid state but WP_DEFAULT_THEME doesn't exist,
+	 * switch to the latest core default theme that's installed.
+	 * If it turns out that this latest core default theme is our current
+	 * theme, then there's nothing we can do about that, so we have to bail,
+	 * rather than going into an infinite loop. (This is why there are
+	 * checks against WP_DEFAULT_THEME above, also.) We also can't do anything
+	 * if it turns out there is no default theme installed. (That's `false`.)
+	 */
+	$default = WP_Theme::get_core_default_theme();
+	if ( false === $default || get_stylesheet() == $default->get_stylesheet() ) {
+		return true;
 	}
 
-	if ( is_child_theme() && ! file_exists( get_stylesheet_directory() . '/style.css' ) ) {
-		switch_theme( WP_DEFAULT_THEME );
-		return false;
-	}
-
-	return true;
+	switch_theme( $default->get_stylesheet() );
+	return false;
 }
 
 /**
diff --git a/wp-includes/version.php b/wp-includes/version.php
index 7cac484ce4..b1477a384b 100644
--- a/wp-includes/version.php
+++ b/wp-includes/version.php
@@ -4,7 +4,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '4.4-beta4-35737';
+$wp_version = '4.4-beta4-35738';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.