diff --git a/lib/rack/ssl.rb b/lib/rack/ssl.rb
index 5e4b94e..c991372 100644
--- a/lib/rack/ssl.rb
+++ b/lib/rack/ssl.rb
@@ -11,6 +11,7 @@ def call(env)
       if scheme(env) == 'https'
         status, headers, body = @app.call(env)
         headers = hsts_headers.merge(headers)
+        flag_cookies_as_secure!(headers)
         [status, headers, body]
       else
         redirect_to_https(env)
@@ -39,5 +40,16 @@ def redirect_to_https(env)
       def hsts_headers
         { 'Strict-Transport-Security' => "max-age=16070400; includeSubDomains" }
       end
+
+      def flag_cookies_as_secure!(headers)
+        cookies = headers['Set-Cookie'].split("\n")
+        headers['Set-Cookie'] = cookies.map { |cookie|
+          if cookie !~ / secure;/
+            "#{cookie}; secure"
+          else
+            cookie
+          end
+        }.join("\n")
+      end
   end
 end
diff --git a/test/test_ssl.rb b/test/test_ssl.rb
index 84c28b0..c6004cb 100644
--- a/test/test_ssl.rb
+++ b/test/test_ssl.rb
@@ -7,7 +7,11 @@ class TestSSL < Test::Unit::TestCase
   include Rack::Test::Methods
 
   def default_app
-    Rack::SSL.new(lambda { |env| [200, {"Content-Type" => "text/html"}, ["OK"]] })
+    Rack::SSL.new(lambda { |env|
+      headers = {'Content-Type' => "text/html"}
+      headers['Set-Cookie'] = "id=1; path=/\ntoken=abc; path=/; secure; HttpOnly"
+      [200, headers, ["OK"]]
+    })
   end
 
   def app
@@ -21,18 +25,26 @@ def test_allows_https_url
   end
 
   def test_allows_https_proxy_header_url
-    get "http://example.org/path?key=value", {}, 'HTTP_X_FORWARDED_PROTO' => "https"
+    get "http://example.org/", {}, 'HTTP_X_FORWARDED_PROTO' => "https"
     assert last_response.ok?
   end
 
   def test_redirects_http_to_https
     get "http://example.org/path?key=value"
     assert last_response.redirect?
-    assert_equal "https://example.org/path?key=value", last_response.headers['Location']
+    assert_equal "https://example.org/path?key=value",
+      last_response.headers['Location']
   end
 
   def test_strict_transport_security_header
-    get "https://example.org/path?key=value"
-    assert_equal "max-age=16070400; includeSubDomains", last_response.headers['Strict-Transport-Security']
+    get "https://example.org/"
+    assert_equal "max-age=16070400; includeSubDomains",
+      last_response.headers['Strict-Transport-Security']
+  end
+
+  def test_flag_cookies_as_secure
+    get "https://example.org/"
+    assert_equal ["id=1; path=/; secure", "token=abc; path=/; secure; HttpOnly"],
+      last_response.headers['Set-Cookie'].split("\n")
   end
 end