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Gerard Butler gave his fans major "300" wood last night by putting on a Spartan soldier's helmet and belting out the line that made him famous.

Gerard was signing autographs outside "Jimmy Kimmel Live" when someone handed him a replica helmet ... much like the one he wore in the 2006 hit film.

Gerard not only signed the memorabilia, but put it on his head ... and screamed ... "This is Sparta!"

Unfortunately, he wasn't shirtless or ridiculously ripped like he was in the movie .. but you gotta see how the fans react.