Rob Gronkowski -- NFL Star Hosts Topless Pool Party

OK, so HE was the one who went topless ... but DAMN.

New England Patriots tight end Rob Gronkowski ditched his shirt ... and donned a pair of d-bag shades ... to host the Hot 100 party at Wet Republic on Vegas this weekend.

Of course, Rob isn't shy about his ridiculous physique ... dude tore off his shirt at a Super Bowl after-party back in February ... a move that was highly scrutinized 'cause his team had just LOST.