diff --git a/actionpack/test/controller/localized_templates_test.rb b/actionpack/test/controller/localized_templates_test.rb
index 5c5cef66d56..d4cd82368e5 100644
--- a/actionpack/test/controller/localized_templates_test.rb
+++ b/actionpack/test/controller/localized_templates_test.rb
@@ -45,4 +45,11 @@ class LocalizedTemplatesTest < ActionController::TestCase
     assert_equal "Ciao Mondo", @response.body
     assert_equal "text/html",  @response.media_type
   end
+
+  def test_use_locale_with_lowdash
+    I18n.locale = :"de_AT"
+
+    get :hello_world
+    assert_equal "Guten Morgen", @response.body
+  end
 end
diff --git a/actionpack/test/fixtures/localized/hello_world.de_AT.html b/actionpack/test/fixtures/localized/hello_world.de_AT.html
new file mode 100644
index 00000000000..3b6e1802611
--- /dev/null
+++ b/actionpack/test/fixtures/localized/hello_world.de_AT.html
@@ -0,0 +1 @@
+Guten Morgen
\ No newline at end of file
diff --git a/actionview/lib/action_view/template/resolver.rb b/actionview/lib/action_view/template/resolver.rb
index b86a365a6b4..60dea1993b2 100644
--- a/actionview/lib/action_view/template/resolver.rb
+++ b/actionview/lib/action_view/template/resolver.rb
@@ -19,7 +19,7 @@ module ActionView
       def build_path_regex
         handlers = Template::Handlers.extensions.map { |x| Regexp.escape(x) }.join("|")
         formats = Template::Types.symbols.map { |x| Regexp.escape(x) }.join("|")
-        locales = "[a-z]{2}(?:-[A-Z]{2})?"
+        locales = "[a-z]{2}(?:[-_][A-Z]{2})?"
         variants = "[^.]*"
 
         %r{
diff --git a/actionview/test/template/resolver_shared_tests.rb b/actionview/test/template/resolver_shared_tests.rb
index 37452defd27..e7f17fbf457 100644
--- a/actionview/test/template/resolver_shared_tests.rb
+++ b/actionview/test/template/resolver_shared_tests.rb
@@ -232,4 +232,14 @@ module ResolverSharedTests
 
     assert_empty context.find_all("hello_world.html", "test", false, [], {})
   end
+
+  def test_finds_template_with_lowdash_format
+    with_file "test/hello_world.es_AR.text.erb", "Texto simple!"
+
+    es_ar = context.find_all("hello_world", "test", false, [], locale: [:es_AR])
+
+    assert_equal 1, es_ar.size
+
+    assert_equal "Texto simple!", es_ar[0].source
+  end
 end