Stephanie Seymour -- TRYING ON LINGERIE ... In Store Window!

She's been naked for Playboy ... and donned itsy-bitsy bikinis for Sports Illustrated ... so it's no wonder Stephanie Seymour wasn't shy when it came to trying on lingerie in Italy yesterday.

The 45-year-old model grabbed some racy outfits while shopping at Agent Provocateur in Milan with her 66-year-old husband Peter Brant ... and tried them on right in front of the store window. It was awesome.

FYI -- Peter Brant is a paper mogul who happens to be a frickin' BILLIONAIRE!!!

So, good for him ... and good for her.