diff --git a/core/includes/install.core.inc b/core/includes/install.core.inc
index bfe5087..00c301e 100644
--- a/core/includes/install.core.inc
+++ b/core/includes/install.core.inc
@@ -730,6 +730,8 @@ function install_tasks($install_state) {
       'run' => $install_state['settings_verified'] ? INSTALL_TASK_SKIP : INSTALL_TASK_RUN_IF_NOT_COMPLETED,
       'function' => 'Drupal\Core\Installer\Form\SiteSettingsForm',
     ),
+    'install_write_profile' => array(
+    ),
     'install_verify_database_ready' => array(
       'run' => $install_state['database_ready'] ? INSTALL_TASK_SKIP : INSTALL_TASK_RUN_IF_NOT_COMPLETED,
     ),
@@ -2263,3 +2265,20 @@ function install_display_requirements($install_state, $requirements) {
     }
   }
 }
+
+/**
+ * Installation task; ensures install profile is written to settings.php.
+ *
+ * @param array $install_state
+ *   An array of information about the current installation state.
+ */
+function install_write_profile($install_state) {
+  if (Settings::get('install_profile') !== $install_state['parameters']['profile']) {
+    // Remember the profile which was used.
+    $settings['settings']['install_profile'] = (object) array(
+      'value' => $install_state['parameters']['profile'],
+      'required' => TRUE,
+    );
+    drupal_rewrite_settings($settings);
+  }
+}
diff --git a/core/modules/system/src/Tests/Installer/InstallerExistingSettingsTest.php b/core/modules/system/src/Tests/Installer/InstallerExistingSettingsNoProfileTest.php
similarity index 74%
copy from core/modules/system/src/Tests/Installer/InstallerExistingSettingsTest.php
copy to core/modules/system/src/Tests/Installer/InstallerExistingSettingsNoProfileTest.php
index 1951f57..61dd5be 100644
--- a/core/modules/system/src/Tests/Installer/InstallerExistingSettingsTest.php
+++ b/core/modules/system/src/Tests/Installer/InstallerExistingSettingsNoProfileTest.php
@@ -2,26 +2,27 @@
 
 /**
  * @file
- * Contains \Drupal\system\Tests\Installer\InstallerExistingSettingsTest.
+ * Contains \Drupal\system\Tests\Installer\InstallerExistingSettingsNoProfileTest.
  */
 
 namespace Drupal\system\Tests\Installer;
 
+use Drupal\Core\Site\Settings;
 use Drupal\simpletest\InstallerTestBase;
 use Drupal\Core\Database\Database;
 
 /**
- * Tests the installer with an existing settings file.
+ * Tests the installer with an existing settings file but no install profile.
  *
  * @group Installer
  */
-class InstallerExistingSettingsTest extends InstallerTestBase {
+class InstallerExistingSettingsNoProfileTest extends InstallerTestBase {
 
   /**
    * {@inheritdoc}
    *
-   * Fully configures a preexisting settings.php file before invoking the
-   * interactive installer.
+   * Configures a preexisting settings.php file without an install_profile
+   * setting before invoking the interactive installer.
    */
   protected function setUp() {
     // Pre-configure hash salt.
@@ -31,15 +32,6 @@ protected function setUp() {
       'required' => TRUE,
     );
 
-    // Actually the install profile should be skipped to because it is written
-    // to settings.php.
-    // @todo https://www.drupal.org/node/2451369 Fix install_profile so that it
-    //   is written to an existing settings.php if possible or if set used.
-    $this->settings['settings']['install_profile'] = (object) array(
-      'value' => 'testing',
-      'required' => TRUE,
-    );
-
     // Pre-configure database credentials.
     $connection_info = Database::getConnectionInfo();
     unset($connection_info['default']['pdo']);
@@ -81,6 +73,7 @@ protected function setUpSettings() {
   public function testInstaller() {
     $this->assertUrl('user/1');
     $this->assertResponse(200);
+    $this->assertEqual('testing', Settings::get('install_profile'));
   }
 
 }
diff --git a/core/modules/system/src/Tests/Installer/InstallerExistingSettingsTest.php b/core/modules/system/src/Tests/Installer/InstallerExistingSettingsTest.php
index 1951f57..25be1ef 100644
--- a/core/modules/system/src/Tests/Installer/InstallerExistingSettingsTest.php
+++ b/core/modules/system/src/Tests/Installer/InstallerExistingSettingsTest.php
@@ -31,12 +31,10 @@ protected function setUp() {
       'required' => TRUE,
     );
 
-    // Actually the install profile should be skipped to because it is written
-    // to settings.php.
-    // @todo https://www.drupal.org/node/2451369 Fix install_profile so that it
-    //   is written to an existing settings.php if possible or if set used.
+    // During interactive install we'll change this to a different profile and
+    // this test will ensure that the new value is written to settings.php.
     $this->settings['settings']['install_profile'] = (object) array(
-      'value' => 'testing',
+      'value' => 'minimal',
       'required' => TRUE,
     );
 
@@ -81,6 +79,7 @@ protected function setUpSettings() {
   public function testInstaller() {
     $this->assertUrl('user/1');
     $this->assertResponse(200);
+    $this->assertEqual('testing', drupal_get_profile(), 'Profile was changed from minimal to testing during interactive install.');
   }
 
 }
diff --git a/sites/default/default.settings.php b/sites/default/default.settings.php
index effa94d..d4bb0de 100644
--- a/sites/default/default.settings.php
+++ b/sites/default/default.settings.php
@@ -256,6 +256,18 @@
  */
 
 /**
+ * The active installation profile.
+ *
+ * Changing this after installation is not recommended as it changes which
+ * directories are scanned during extension discovery. If this is set prior to
+ * installation this value will be rewritten according to the profile selected
+ * by the user.
+ *
+ * @see install_select_profile()
+ */
+# $settings['install_profile'] = '';
+
+/**
  * Salt for one-time login links, cancel links, form tokens, etc.
  *
  * This variable will be set to a random value by the installer. All one-time