diff --git a/lib/rack/utf8_sanitizer.rb b/lib/rack/utf8_sanitizer.rb
index eea3f12..5e50424 100644
--- a/lib/rack/utf8_sanitizer.rb
+++ b/lib/rack/utf8_sanitizer.rb
@@ -2,6 +2,7 @@
 
 require 'uri'
 require 'stringio'
+require 'rack/request'
 
 module Rack
   class UTF8Sanitizer
@@ -126,6 +127,10 @@ def sanitize_rack_input(env)
         end
       end
       return unless @sanitizable_content_types.any? {|type| content_type == type }
+
+      charset = Rack::Request.new(env).content_charset
+      return if charset && charset.downcase != 'utf-8'
+
       uri_encoded = URI_ENCODED_CONTENT_TYPES.any? {|type| content_type == type}
 
       if env['rack.input']
diff --git a/test/test_utf8_sanitizer.rb b/test/test_utf8_sanitizer.rb
index f0240c2..cb055c8 100644
--- a/test/test_utf8_sanitizer.rb
+++ b/test/test_utf8_sanitizer.rb
@@ -252,6 +252,18 @@ def read
       end
     end
 
+    it "sanitizes the rack body if the charset is present and utf-8" do
+      input =  "name=#{CGI.escape("まつもと")}"
+      @rack_input = StringIO.new input
+
+      env = request_env.update('CONTENT_TYPE' => "application/x-www-form-urlencoded; charset=utf-8")
+      sanitize_form_data(env) do |sanitized_input|
+        sanitized_input.encoding.should == Encoding::UTF_8
+        sanitized_input.should.be.valid_encoding
+        sanitized_input.should == input
+      end
+    end
+
     it "strip UTF-8 BOM from StringIO rack.input" do
       input = %(\xef\xbb\xbf{"Hello": "World"})
       @rack_input = StringIO.new input
@@ -327,6 +339,18 @@ def read
       end
     end
 
+    it "does not sanitize the rack body if the charset is present and not utf-8" do
+      input = "name=".encode("Shift_JIS") + CGI.escape("まつもと".encode("Shift_JIS", "UTF-8"))
+      @rack_input = StringIO.new input
+
+      env = request_env.update('CONTENT_TYPE' => "application/x-www-form-urlencoded; charset=Shift_JIS")
+      sanitize_form_data(env) do |sanitized_input|
+        sanitized_input.encoding.should == Encoding::SHIFT_JIS
+        sanitized_input.should.be.valid_encoding
+        sanitized_input.should == input
+      end
+    end
+
     it "adjusts content-length when replacing input" do
       input =  "foo=bla&quux=bar\xED"
       @rack_input = StringIO.new input