diff --git a/lib/sinatra/indifferent_hash.rb b/lib/sinatra/indifferent_hash.rb
index 08126bba..9d54e317 100644
--- a/lib/sinatra/indifferent_hash.rb
+++ b/lib/sinatra/indifferent_hash.rb
@@ -146,6 +146,17 @@ module Sinatra
       super(other_hash.is_a?(self.class) ? other_hash : self.class[other_hash])
     end
 
+    if method_defined?(:transform_values!) # Added in Ruby 2.4
+      def transform_values(&block)
+        dup.transform_values!(&block)
+      end
+
+      def transform_values!
+        super
+        super(&method(:convert_value))
+      end
+    end
+
     private
 
     def convert_key(key)
diff --git a/test/indifferent_hash_test.rb b/test/indifferent_hash_test.rb
index 69101b48..2ceb65d5 100644
--- a/test/indifferent_hash_test.rb
+++ b/test/indifferent_hash_test.rb
@@ -205,4 +205,22 @@ class TestIndifferentHash < Minitest::Test
     @hash.replace(?a=>1, :q=>2)
     assert_equal({ ?a=>1, ?q=>2 }, @hash)
   end
+
+  def test_transform_values!
+    skip_if_lacking :transform_values!
+
+    @hash.transform_values! { |v| v.is_a?(Hash) ? Hash[v.to_a] : v }
+
+    assert_instance_of Sinatra::IndifferentHash, @hash[:simple_nested]
+  end
+
+  def test_transform_values
+    skip_if_lacking :transform_values
+
+    hash2 = @hash.transform_values { |v| v.respond_to?(:upcase) ? v.upcase : v }
+
+    refute_equal @hash, hash2
+    assert_equal :A, hash2[:a]
+    assert_equal :A, hash2[?a]
+  end
 end