diff --git a/core/lib/Drupal/Core/Config/ConfigInstaller.php b/core/lib/Drupal/Core/Config/ConfigInstaller.php
index 7209030..ae12c91 100644
--- a/core/lib/Drupal/Core/Config/ConfigInstaller.php
+++ b/core/lib/Drupal/Core/Config/ConfigInstaller.php
@@ -112,13 +112,13 @@ public function installDefaultConfig($type, $name) {
         $prefix = $name . '.';
       }
 
-      // Gets a profile storage to search for overrides if necessary.
-      $profile_storage = $this->getProfileStorage($name);
+      // Gets profile storages to search for overrides if necessary.
+      $profile_storages = $this->getProfileStorages($name);
 
       // Gather information about all the supported collections.
       $collection_info = $this->configManager->getConfigCollectionInfo();
       foreach ($collection_info->getCollectionNames() as $collection) {
-        $config_to_create = $this->getConfigToCreate($storage, $collection, $prefix, $profile_storage);
+        $config_to_create = $this->getConfigToCreate($storage, $collection, $prefix, $profile_storages);
         // If we're installing a profile ensure configuration that is overriding
         // is excluded.
         if ($name == $this->drupalGetProfile()) {
@@ -223,19 +223,22 @@ public function installOptionalConfig(StorageInterface $storage = NULL, $depende
    *  The configuration collection to use.
    * @param string $prefix
    *   (optional) Limit to configuration starting with the provided string.
+   * @param \Drupal\Core\Config\StorageInterface[] $profile_storages
+   *   An array of storage interfaces containing profile configuration to check
+   *   for overrides.
    *
    * @return array
    *   An array of configuration data read from the source storage keyed by the
    *   configuration object name.
    */
-  protected function getConfigToCreate(StorageInterface $storage, $collection, $prefix = '', StorageInterface $profile_storage = NULL) {
+  protected function getConfigToCreate(StorageInterface $storage, $collection, $prefix = '', array $profile_storages = []) {
     if ($storage->getCollectionName() != $collection) {
       $storage = $storage->createCollection($collection);
     }
     $data = $storage->readMultiple($storage->listAll($prefix));
 
     // Check to see if the corresponding override storage has any overrides.
-    if ($profile_storage) {
+    foreach ($profile_storages as $profile_storage) {
       if ($profile_storage->getCollectionName() != $collection) {
         $profile_storage = $profile_storage->createCollection($collection);
       }
@@ -435,11 +438,11 @@ public function checkConfigurationToInstall($type, $name) {
     $enabled_extensions = $this->getEnabledExtensions();
     // Add the extension that will be enabled to the list of enabled extensions.
     $enabled_extensions[] = $name;
-    // Gets a profile storage to search for overrides if necessary.
-    $profile_storage = $this->getProfileStorage($name);
+    // Gets profile storages to search for overrides if necessary.
+    $profile_storages = $this->getProfileStorages($name);
 
     // Check the dependencies of configuration provided by the module.
-    $invalid_default_config = $this->findDefaultConfigWithUnmetDependencies($storage, $enabled_extensions, $profile_storage);
+    $invalid_default_config = $this->findDefaultConfigWithUnmetDependencies($storage, $enabled_extensions, $profile_storages);
     if (!empty($invalid_default_config)) {
       throw UnmetDependenciesException::create($name, $invalid_default_config);
     }
@@ -460,14 +463,19 @@ public function checkConfigurationToInstall($type, $name) {
   /**
    * Finds default configuration with unmet dependencies.
    *
+   * @param \Drupal\Core\Config\StorageInterface $storage
+   *   The storage containing the default configuration.
    * @param array $enabled_extensions
    *   A list of all the currently enabled modules and themes.
+   * @param \Drupal\Core\Config\StorageInterface[] $profile_storages
+   *   An array of storage interfaces containing profile configuration to check
+   *   for overrides.
    *
    * @return array
    *   List of configuration that has unmet dependencies
    */
-  protected function findDefaultConfigWithUnmetDependencies(StorageInterface $storage, array $enabled_extensions, StorageInterface $profile_storage = NULL) {
-    $config_to_create = $this->getConfigToCreate($storage, StorageInterface::DEFAULT_COLLECTION, '', $profile_storage);
+  protected function findDefaultConfigWithUnmetDependencies(StorageInterface $storage, array $enabled_extensions, array $profile_storages = []) {
+    $config_to_create = $this->getConfigToCreate($storage, StorageInterface::DEFAULT_COLLECTION, '', $profile_storages);
     $all_config = array_merge($this->configFactory->listAll(), array_keys($config_to_create));
     return array_filter(array_keys($config_to_create), function($config_name) use ($enabled_extensions, $all_config, $config_to_create) {
       return !$this->validateDependencies($config_name, $config_to_create[$config_name], $enabled_extensions, $all_config);
@@ -550,27 +558,31 @@ protected function getEnabledExtensions() {
   /**
    * Gets the profile storage to use to check for profile overrides.
    *
+   * The install profile can override module configuration during a module
+   * install. Both the install and optional directories are checked for matching
+   * configuration. This allows profiles to override default configuration for
+   * modules they do not depend on.
+   *
    * @param string $installing_name
    *   (optional) The name of the extension currently being installed.
    *
-   * @return \Drupal\Core\Config\StorageInterface|null
-   *   A storage to access configuration from the installation profile. If a
-   *   Drupal installation is not in progress or we're installing the profile
-   *   itself, then it will return NULL as the profile storage should not be
-   *   used.
+   * @return \Drupal\Core\Config\StorageInterface[]|null
+   *   Storages to access configuration from the installation profile. If we're
+   *   installing the profile itself, then it will return an empty array as the
+   *   profile storage should not be used.
    */
-  protected function getProfileStorage($installing_name = '') {
+  protected function getProfileStorages($installing_name = '') {
     $profile = $this->drupalGetProfile();
-    if ($this->drupalInstallationAttempted() && $profile != $installing_name) {
-      // Profiles should not contain optional configuration so always use the
-      // install directory.
-      $profile_install_path = $this->getDefaultConfigDirectory('module', $profile);
-      $profile_storage = new FileStorage($profile_install_path, StorageInterface::DEFAULT_COLLECTION);
-    }
-    else {
-      $profile_storage = NULL;
+    $profile_storages = [];
+    if ($profile && $profile != $installing_name) {
+      $profile_path = $this->drupalGetPath('module', $profile);
+      foreach ([InstallStorage::CONFIG_INSTALL_DIRECTORY, InstallStorage::CONFIG_OPTIONAL_DIRECTORY] as $directory) {
+        if (is_dir($profile_path . '/' . $directory)) {
+          $profile_storages[] = new FileStorage($profile_path . '/' . $directory, StorageInterface::DEFAULT_COLLECTION);
+        }
+      }
     }
-    return $profile_storage;
+    return $profile_storages;
   }
 
   /**
diff --git a/core/modules/config/src/Tests/ConfigInstallProfileOverrideTest.php b/core/modules/config/src/Tests/ConfigInstallProfileOverrideTest.php
index b5409be..e10dfe0 100644
--- a/core/modules/config/src/Tests/ConfigInstallProfileOverrideTest.php
+++ b/core/modules/config/src/Tests/ConfigInstallProfileOverrideTest.php
@@ -92,12 +92,12 @@ function testInstallProfileConfigOverwrite() {
     }
 
     // Install the config_test module and ensure that the override from the
-    // install profile is not used. Optional configuration can not override
+    // install profile is used. Optional configuration can override
     // configuration in a modules config/install directory.
     $this->container->get('module_installer')->install(['config_test']);
     $this->rebuildContainer();
     $config_test_storage = \Drupal::entityManager()->getStorage('config_test');
-    $this->assertEqual($config_test_storage->load('dotted.default')->label(), 'Default', 'The config_test entity is not overridden by the profile optional configuration.');
+    $this->assertEqual($config_test_storage->load('dotted.default')->label(), 'Default install profile override', 'The config_test entity is overridden by the profile optional configuration.');
     // Test that override of optional configuration does work.
     $this->assertEqual($config_test_storage->load('override')->label(), 'Override', 'The optional config_test entity is overridden by the profile optional configuration.');
     // Test that override of optional configuration which introduces an unmet
diff --git a/core/modules/locale/src/Tests/LocaleConfigTranslationImportTest.php b/core/modules/locale/src/Tests/LocaleConfigTranslationImportTest.php
index b0450e5..dbe8310 100644
--- a/core/modules/locale/src/Tests/LocaleConfigTranslationImportTest.php
+++ b/core/modules/locale/src/Tests/LocaleConfigTranslationImportTest.php
@@ -78,6 +78,10 @@ public function testConfigTranslationModuleInstall() {
     $this->container->get('module_installer')->install(['block', 'config_translation']);
     $this->resetAll();
 
+    // The testing profile overrides locale.settings to disable translation
+    // import. Test that this override is in place.
+    $this->assertFalse($this->config('locale.settings')->get('translation.import_enabled'), 'Translations imports are disabled by default in the Testing profile.');
+
     $admin_user = $this->drupalCreateUser(array('administer modules', 'administer site configuration', 'administer languages', 'access administration pages', 'administer permissions', 'translate configuration'));
     $this->drupalLogin($admin_user);