Talk about an epic 40th birthday party!  Not only did Reese Witherspoon look stunning at her star-studded party, but she took the stage at one point to sing "Sweet Home Alabama" and had a very special guitar player backing her up!

The one and only Keith Urban!

Here's a clip -

Click here to check out more pics from Reese's party!  Other stars at the party included Taylor Swift, Laura Dern, and Kate Hudson!!