Justin Bieber at Cannes Party -- Dressed to ... What's the Opposite of Impress?

Justin Bieber strolled into a fancy formal party in Cannes ... but he was dressed for a backyard barbecue on the Florida panhandle.

Bieber was a guest at Roberto Cavalli's (see below) annual Cannes yacht party, where everyone was dressed for a high-end evening ... except for the Biebs.

While John Travolta showed up in a tux, Bieber opted for a denim vest with a jersey, a pair of baggy shorts, super casual slippers, and no socks.

On a positive note ... he was wearing a shirt.

On a different note -- Justin offered this review of his outfit ... "I know I look fly but s***."