diff --git a/Yesod/Core.hs b/Yesod/Core.hs
--- a/Yesod/Core.hs
+++ b/Yesod/Core.hs
@@ -41,6 +41,7 @@
     , logOtherS
       -- * Sessions
     , SessionBackend (..)
+    , customizeSessionCookies
     , defaultClientSessionBackend
     , clientSessionBackend
     , clientSessionDateCacher
diff --git a/Yesod/Core/Class/Yesod.hs b/Yesod/Core/Class/Yesod.hs
--- a/Yesod/Core/Class/Yesod.hs
+++ b/Yesod/Core/Class/Yesod.hs
@@ -12,7 +12,7 @@
 
 import           Blaze.ByteString.Builder           (Builder)
 import           Blaze.ByteString.Builder.Char.Utf8 (fromText)
-import           Control.Arrow                      ((***))
+import           Control.Arrow                      ((***), second)
 import           Control.Monad                      (forM, when, void)
 import           Control.Monad.IO.Class             (MonadIO (liftIO))
 import           Control.Monad.Logger               (LogLevel (LevelInfo, LevelOther),
@@ -235,7 +235,10 @@
     jsLoader :: site -> ScriptLoadPosition site
     jsLoader _ = BottomOfBody
 
-    -- | Create a session backend. Returning `Nothing' disables sessions.
+    -- | Create a session backend. Returning 'Nothing' disables
+    -- sessions. If you'd like to change the way that the session
+    -- cookies are created, take a look at
+    -- 'customizeSessionCookies'.
     --
     -- Default: Uses clientsession with a 2 hour timeout.
     makeSessionBackend :: site -> IO (Maybe SessionBackend)
@@ -531,6 +534,33 @@
         , LS $ fileLocationToString loc
         , LB ")\n"
         ]
+
+
+-- | Customize the cookies used by the session backend.  You may
+-- use this function on your definition of 'makeSessionBackend'.
+--
+-- For example, you could set the cookie domain so that it
+-- would work across many subdomains:
+--
+-- @
+-- makeSessionBackend = fmap (customizeSessionCookie addDomain) ...
+--   where
+--     addDomain cookie = cookie { 'setCookieDomain' = Just \".example.com\" }
+-- @
+--
+-- Default: Do not customize anything ('id').
+customizeSessionCookies :: (SetCookie -> SetCookie) -> (SessionBackend -> SessionBackend)
+customizeSessionCookies customizeCookie backend = backend'
+  where
+    customizeHeader (AddCookie cookie) = AddCookie (customizeCookie cookie)
+    customizeHeader other              = other
+    customizeSaveSession = (fmap . fmap . fmap) customizeHeader
+    backend' =
+      backend {
+        sbLoadSession = \req ->
+          second customizeSaveSession `fmap` sbLoadSession backend req
+      }
+
 
 defaultClientSessionBackend :: Int -- ^ minutes
                             -> FilePath -- ^ key file
diff --git a/yesod-core.cabal b/yesod-core.cabal
--- a/yesod-core.cabal
+++ b/yesod-core.cabal
@@ -1,5 +1,5 @@
 name:            yesod-core
-version:         1.2.0.2
+version:         1.2.0.3
 license:         MIT
 license-file:    LICENSE
 author:          Michael Snoyman <michael@snoyman.com>