diff --git a/actionpack/CHANGELOG.md b/actionpack/CHANGELOG.md
index fd58f00e83..d63e5c4d6e 100644
--- a/actionpack/CHANGELOG.md
+++ b/actionpack/CHANGELOG.md
@@ -1,3 +1,8 @@
+*   Use `String#bytesize` instead of `String#size` when checking for cookie
+    overflow.
+
+    *Agis Anastasopoulos*
+   
 *   `render nothing: true` or rendering a `nil` body no longer add a single
     space to the response body.
 
diff --git a/actionpack/lib/action_dispatch/middleware/cookies.rb b/actionpack/lib/action_dispatch/middleware/cookies.rb
index e069840b8e..ac9e5effe2 100644
--- a/actionpack/lib/action_dispatch/middleware/cookies.rb
+++ b/actionpack/lib/action_dispatch/middleware/cookies.rb
@@ -468,7 +468,7 @@ module ActionDispatch
           options = { :value => @verifier.generate(serialize(name, options)) }
         end
 
-        raise CookieOverflow if options[:value].size > MAX_COOKIE_SIZE
+        raise CookieOverflow if options[:value].bytesize > MAX_COOKIE_SIZE
         @parent_jar[name] = options
       end
 
@@ -526,7 +526,7 @@ module ActionDispatch
 
         options[:value] = @encryptor.encrypt_and_sign(serialize(name, options[:value]))
 
-        raise CookieOverflow if options[:value].size > MAX_COOKIE_SIZE
+        raise CookieOverflow if options[:value].bytesize > MAX_COOKIE_SIZE
         @parent_jar[name] = options
       end