diff --git a/actionview/lib/action_view/helpers/tag_helper.rb b/actionview/lib/action_view/helpers/tag_helper.rb
index ffb364dce1..b7f8661f2e 100644
--- a/actionview/lib/action_view/helpers/tag_helper.rb
+++ b/actionview/lib/action_view/helpers/tag_helper.rb
@@ -94,7 +94,7 @@ module ActionView
         def tag_option(key, value, escape)
           case value
           when Array, Hash
-            value = build_tag_values(value) if key.to_s == "class"
+            value = TagHelper.build_tag_values(value) if key.to_s == "class"
             value = escape ? safe_join(value, " ") : value.join(" ")
           else
             value = escape ? ERB::Util.unwrapped_html_escape(value) : value.to_s
@@ -104,25 +104,6 @@ module ActionView
         end
 
         private
-          def build_tag_values(*args)
-            tag_values = []
-
-            args.each do |tag_value|
-              case tag_value
-              when Hash
-                tag_value.each do |key, val|
-                  tag_values << key if val
-                end
-              when Array
-                tag_values << build_tag_values(*tag_value).presence
-              else
-                tag_values << tag_value.to_s if tag_value.present?
-              end
-            end
-
-            tag_values.compact.flatten
-          end
-
           def prefix_tag_option(prefix, key, value, escape)
             key = "#{prefix}-#{key.to_s.dasherize}"
             unless value.is_a?(String) || value.is_a?(Symbol) || value.is_a?(BigDecimal)
@@ -309,6 +290,19 @@ module ActionView
         end
       end
 
+      # Returns a string of class names built from +args+.
+      #
+      # ==== Examples
+      #   class_names("foo", "bar")
+      #    # => "foo bar"
+      #   class_names({ foo: true, bar: false })
+      #    # => "foo"
+      #   class_names(nil, false, 123, "", "foo", { bar: true })
+      #    # => "foo bar"
+      def class_names(*args)
+        safe_join(build_tag_values(*args), " ")
+      end
+
       # Returns a CDATA section with the given +content+. CDATA sections
       # are used to escape blocks of text containing characters which would
       # otherwise be recognized as markup. CDATA sections begin with the string
@@ -339,6 +333,26 @@ module ActionView
       end
 
       private
+        def build_tag_values(*args)
+          tag_values = []
+
+          args.each do |tag_value|
+            case tag_value
+            when Hash
+              tag_value.each do |key, val|
+                tag_values << key if val
+              end
+            when Array
+              tag_values << build_tag_values(*tag_value).presence
+            else
+              tag_values << tag_value.to_s if tag_value.present?
+            end
+          end
+
+          tag_values.compact.flatten
+        end
+        module_function :build_tag_values
+
         def tag_builder
           @tag_builder ||= TagBuilder.new(self)
         end
diff --git a/actionview/test/template/tag_helper_test.rb b/actionview/test/template/tag_helper_test.rb
index 44496bce9b..2c800ff96c 100644
--- a/actionview/test/template/tag_helper_test.rb
+++ b/actionview/test/template/tag_helper_test.rb
@@ -338,6 +338,17 @@ class TagHelperTest < ActionView::TestCase
     assert_equal "<p class=\"song play>\">limelight</p>", str
   end
 
+  def test_class_names
+    assert_equal "song play", class_names(["song", { "play": true }])
+    assert_equal "song", class_names({ "song": true, "play": false })
+    assert_equal "song", class_names([{ "song": true }, { "play": false }])
+    assert_equal "song", class_names({ song: true, play: false })
+    assert_equal "song", class_names([{ song: true }, nil, false])
+    assert_equal "song", class_names(["song", { foo: false }])
+    assert_equal "song play", class_names({ "song": true, "play": true })
+    assert_equal "", class_names({ "song": false, "play": false })
+  end
+
   def test_content_tag_with_data_attributes
     assert_dom_equal '<p data-number="1" data-string="hello" data-string-with-quotes="double&quot;quote&quot;party&quot;">limelight</p>',
       content_tag("p", "limelight", data: { number: 1, string: "hello", string_with_quotes: 'double"quote"party"' })