diff --git a/railties/lib/generators/rails/app/app_generator.rb b/railties/lib/generators/rails/app/app_generator.rb
index 92e0d37436..888e3b2ddd 100644
--- a/railties/lib/generators/rails/app/app_generator.rb
+++ b/railties/lib/generators/rails/app/app_generator.rb
@@ -220,6 +220,8 @@ module Rails::Generators
           raise Error, "Invalid application name #{app_name}. Please give a name which does not start with numbers."
         elsif RESERVED_NAMES.include?(app_name)
           raise Error, "Invalid application name #{app_name}. Please give a name which does not match one of the reserved rails words."
+        elsif Object.const_defined?(app_const_base)
+          raise Error, "Invalid application name #{app_name}, constant #{app_const_base} is already in use. Please choose another application name."
         end
       end
 
diff --git a/railties/test/generators/app_generator_test.rb b/railties/test/generators/app_generator_test.rb
index 412034029e..1303adfaab 100644
--- a/railties/test/generators/app_generator_test.rb
+++ b/railties/test/generators/app_generator_test.rb
@@ -65,6 +65,13 @@ class AppGeneratorTest < Rails::Generators::TestCase
     assert_equal "Invalid application name 43-things. Please give a name which does not start with numbers.\n", content
   end
 
+  def test_application_name_raises_an_error_if_name_already_used_constant
+    %w{ String Hash Class Module Set Symbol }.each do |ruby_class|
+      content = capture(:stderr){ run_generator [File.join(destination_root, ruby_class)] }
+      assert_equal "Invalid application name #{ruby_class}, constant #{ruby_class} is already in use. Please choose another application name.\n", content
+    end
+  end
+
   def test_invalid_application_name_is_fixed
     run_generator [File.join(destination_root, "things-43")]
     assert_file "things-43/config/environment.rb", /Things43::Application\.initialize!/