diff --git a/framework/CHANGELOG.md b/framework/CHANGELOG.md
index 6cad5c4..3a6a4a0 100644
--- a/framework/CHANGELOG.md
+++ b/framework/CHANGELOG.md
@@ -35,6 +35,7 @@ Yii Framework 2 Change Log
 - Bug #11973: Fixed `yii\helpers\BaseHtml::getAttributeValue()` to work with `items[]` notation correctly (silverfire)
 - Bug #12100: Fixed `yii\filters\HttpCache` was sending an empty Pragma header (sergeymakinen)
 - Bug #12107: Fixed REST Serializer to validate input for 'expand' and 'fields' parameter, which crashed on array input (njspok, cebe)
+- Bug #12152: Fixed BC break for `UniqueValidator` custom message when validating multiple attributes (rob006)
 - Enh #12230: Allows BaseHtml::activeListInput to override the field value (RangelReale)
 
 
diff --git a/framework/validators/UniqueValidator.php b/framework/validators/UniqueValidator.php
index eb45f6e..5ffee33 100644
--- a/framework/validators/UniqueValidator.php
+++ b/framework/validators/UniqueValidator.php
@@ -60,12 +60,24 @@ class UniqueValidator extends Validator
      */
     public $filter;
     /**
-     * @var string the user-defined error message used when [[targetAttribute]] is an array. It may contain the following placeholders:
+     * @var string the user-defined error message. When validating single attribute, it may contain
+     * the following placeholders which will be replaced accordingly by the validator:
+     *
+     * - `{attribute}`: the label of the attribute being validated
+     * - `{value}`: the value of the attribute being validated
+     *
+     * When validating mutliple attributes, it may contain the following placeholders:
      *
      * - `{attributes}`: the labels of the attributes being validated.
      * - `{values}`: the values of the attributes being validated.
      *
+     */
+    public $message;
+    /**
+     * @var string
      * @since 2.0.9
+     * @deprecated Deprecated since version 2.0.10, to be removed in 2.1. Use [[message]] property
+     * to setup custom message for multiple target attributes.
      */
     public $comboNotUnique;
 
@@ -76,11 +88,18 @@ class UniqueValidator extends Validator
     public function init()
     {
         parent::init();
-        if ($this->message === null) {
-            $this->message = Yii::t('yii', '{attribute} "{value}" has already been taken.');
+        if ($this->message !== null) {
+            return;
         }
-        if ($this->comboNotUnique === null) {
-            $this->comboNotUnique = Yii::t('yii', 'The combination {values} of {attributes} has already been taken.');
+        if (is_array($this->targetAttribute) && count($this->targetAttribute) > 1) {
+            // fallback for deprecated `comboNotUnique` property - use it as message if is set
+            if ($this->comboNotUnique === null) {
+                $this->message = Yii::t('yii', 'The combination {values} of {attributes} has already been taken.');
+            } else {
+                $this->message = $this->comboNotUnique;
+            }
+        } else {
+            $this->message = Yii::t('yii', '{attribute} "{value}" has already been taken.');
         }
     }
 
@@ -153,7 +172,7 @@ class UniqueValidator extends Validator
             }
         }
     }
-    
+
     /**
      * Builds and adds [[comboNotUnique]] error message to the specified model attribute.
      * @param \yii\base\Model $model the data model.
@@ -172,7 +191,7 @@ class UniqueValidator extends Validator
                 $valueCombo[] = '"' . $model->$key . '"';
             }
         }
-        $this->addError($model, $attribute, $this->comboNotUnique, [
+        $this->addError($model, $attribute, $this->message, [
             'attributes' => Inflector::sentence($attributeCombo),
             'values' => implode('-', $valueCombo)
         ]);