diff --git a/admin/settings/location.php b/admin/settings/location.php
index 85d58388ba7..5a4cc9010c8 100644
--- a/admin/settings/location.php
+++ b/admin/settings/location.php
@@ -49,8 +49,8 @@ if ($hassiteconfig) {
         $temp->add(new admin_setting_configtext('googlemapkey3', new lang_string('googlemapkey3', 'core_admin'),
             new lang_string('googlemapkey3_help', 'core_admin'), '', PARAM_RAW, 60));
 
-        $temp->add(new admin_setting_configtext('allcountrycodes', new lang_string('allcountrycodes', 'core_admin'),
-            new lang_string('configallcountrycodes', 'core_admin'), '', '/^(?:\w+(?:,\w+)*)?$/'));
+        $temp->add(new admin_setting_countrycodes('allcountrycodes', new lang_string('allcountrycodes', 'core_admin'),
+            new lang_string('configallcountrycodes', 'core_admin')));
     }
 
     $ADMIN->add('location', $temp);
diff --git a/admin/tests/behat/invalid_allcountrycodes.feature b/admin/tests/behat/invalid_allcountrycodes.feature
new file mode 100644
index 00000000000..399712f45e1
--- /dev/null
+++ b/admin/tests/behat/invalid_allcountrycodes.feature
@@ -0,0 +1,29 @@
+@core @core_admin
+Feature: Administrator is warned and when trying to set invalid allcountrycodes value.
+  In order to avoid misconfiguration of the country selector fields
+  As an admin
+  I want to be warned when I try to set an invalid country code in the allcountrycodes field
+
+  Scenario: Attempting to set allcountrycodes field with valid country codes
+    Given I log in as "admin"
+    And I navigate to "Location > Location settings" in site administration
+    When I set the following administration settings values:
+      | All country codes | CZ,BE,GB,ES |
+    Then I should not see "Invalid country code"
+
+  Scenario: Attempting to set allcountrycodes field with invalid country code
+    Given I log in as "admin"
+    And I navigate to "Location > Location settings" in site administration
+    When I set the following administration settings values:
+      | All country codes | CZ,BE,FOOBAR,GB,ES |
+    Then I should see "Invalid country code: FOOBAR"
+
+  Scenario: Attempting to unset allcountrycodes field
+    Given I log in as "admin"
+    And I navigate to "Location > Location settings" in site administration
+    And I set the following administration settings values:
+      | All country codes | CZ,BE,GB,ES |
+    And I navigate to "Location > Location settings" in site administration
+    When I set the following administration settings values:
+      | All country codes | |
+    Then I should not see "Invalid country code"
diff --git a/lang/en/error.php b/lang/en/error.php
index e41176cdbd1..21416150fc3 100644
--- a/lang/en/error.php
+++ b/lang/en/error.php
@@ -324,6 +324,7 @@ $string['invalidcourselevel'] = 'Incorrect context level';
 $string['invalidcourseformat'] = 'Invalid course format';
 $string['invalidcoursemodule'] = 'Invalid course module ID';
 $string['invalidcoursenameshort'] = 'Invalid short course name';
+$string['invalidcountrycode'] = 'Invalid country code: {$a}';
 $string['invaliddata'] = 'Data submitted is invalid';
 $string['invaliddatarootpermissions'] = 'Invalid permissions detected when trying to create a directory. Turn debugging on for further details.';
 $string['invaliddevicetype'] = 'Invalid device type';
diff --git a/lib/adminlib.php b/lib/adminlib.php
index 9b7445d6b80..29889c777c0 100644
--- a/lib/adminlib.php
+++ b/lib/adminlib.php
@@ -4901,6 +4901,66 @@ class admin_setting_langlist extends admin_setting_configtext {
 }
 
 
+/**
+ * Allows to specify comma separated list of known country codes.
+ *
+ * This is a simple subclass of the plain input text field with added validation so that all the codes are actually
+ * known codes.
+ *
+ * @package     core
+ * @category    admin
+ * @copyright   2020 David Mudrák <david@moodle.com>
+ * @license     https://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
+ */
+class admin_setting_countrycodes extends admin_setting_configtext {
+
+    /**
+     * Construct the instance of the setting.
+     *
+     * @param string $name Name of the admin setting such as 'allcountrycodes' or 'myplugin/countries'.
+     * @param lang_string|string $visiblename Language string with the field label text.
+     * @param lang_string|string $description Language string with the field description text.
+     * @param string $defaultsetting Default value of the setting.
+     * @param int $size Input text field size.
+     */
+    public function __construct($name, $visiblename, $description, $defaultsetting = '', $size = null) {
+        parent::__construct($name, $visiblename, $description, $defaultsetting, '/^(?:\w+(?:,\w+)*)?$/', $size);
+    }
+
+    /**
+     * Validate the setting value before storing it.
+     *
+     * The value is first validated through custom regex so that it is a word consisting of letters, numbers or underscore; or
+     * a comma separated list of such words.
+     *
+     * @param string $data Value inserted into the setting field.
+     * @return bool|string True if the value is OK, error string otherwise.
+     */
+    public function validate($data) {
+
+        $parentcheck = parent::validate($data);
+
+        if ($parentcheck !== true) {
+            return $parentcheck;
+        }
+
+        if ($data === '') {
+            return true;
+        }
+
+        $allcountries = get_string_manager()->get_list_of_countries(true);
+
+        foreach (explode(',', $data) as $code) {
+            if (!isset($allcountries[$code])) {
+                return get_string('invalidcountrycode', 'core_error', $code);
+            }
+        }
+
+        return true;
+    }
+}
+
+
 /**
  * Selection of one of the recognised countries using the list
  * returned by {@link get_list_of_countries()}.