diff --git a/activerecord/CHANGELOG.md b/activerecord/CHANGELOG.md
index 1e035c11e0..81e48862d4 100644
--- a/activerecord/CHANGELOG.md
+++ b/activerecord/CHANGELOG.md
@@ -1,3 +1,9 @@
+*   Fix bug when validating the uniqueness of an aliased attribute.
+
+    Fixes #12402.
+
+    *Lauro Caetano*
+
 *   Update counter cache on a has_many relationship regardless of default scope
 
     Fix #12952.
diff --git a/activerecord/lib/active_record/validations/uniqueness.rb b/activerecord/lib/active_record/validations/uniqueness.rb
index 38f37f5c8a..e2b132ca81 100644
--- a/activerecord/lib/active_record/validations/uniqueness.rb
+++ b/activerecord/lib/active_record/validations/uniqueness.rb
@@ -51,7 +51,15 @@ def build_relation(klass, table, attribute, value) #:nodoc:
           value = value.attributes[reflection.primary_key_column.name] unless value.nil?
         end
 
-        column = klass.columns_hash[attribute.to_s]
+        attribute_name = attribute.to_s
+
+        # the attribute may be an aliased attribute
+        if klass.attribute_aliases[attribute_name]
+          attribute = klass.attribute_aliases[attribute_name]
+          attribute_name = attribute.to_s
+        end
+
+        column = klass.columns_hash[attribute_name]
         value  = klass.connection.type_cast(value, column)
         value  = value.to_s[0, column.limit] if value && column.limit && column.text?
 
diff --git a/activerecord/test/cases/validations/uniqueness_validation_test.rb b/activerecord/test/cases/validations/uniqueness_validation_test.rb
index 71cb9d7b1e..74c696c858 100644
--- a/activerecord/test/cases/validations/uniqueness_validation_test.rb
+++ b/activerecord/test/cases/validations/uniqueness_validation_test.rb
@@ -63,6 +63,14 @@ def test_validate_uniqueness
     assert t2.save, "Should now save t2 as unique"
   end
 
+  def test_validate_uniqueness_with_alias_attribute
+    Topic.alias_attribute :new_title, :title
+    Topic.validates_uniqueness_of(:new_title)
+
+    topic = Topic.new(new_title: 'abc')
+    assert topic.valid?
+  end
+
   def test_validates_uniqueness_with_nil_value
     Topic.validates_uniqueness_of(:title)