Gerard Butler Goofs Around With A Spartan Helmet

King Leonidas is back ... and he's wearing a suit and tie! Wonder if he dressed like this for Brandi Glanville during their wild romp?

Gerard Butler appeared on Jimmy Kimmel Live! last night, and we love that he's such a good sport with his fans! After signing his autograph on a Spartan helmet, Gerard relived his 300 days and put the piece of armor on his head and yelled, "This is Sparta!"

Check out the video below!