'Thor' Star Jaimie Alexander ALMOST NAKED ... at Movie Premiere

11/5/2013 7:06 AM PST

So ... Jaimie Alexander won the "Thor" premiere last night.

The 29-year-old actress -- who plays Lady Sif in the movie -- rolled up to the El Capitan Theater in Hollywood wearing a dress that BARELY covered her lady junk.

Asgard ... meet Ass-guard.