U.S. Open Champ Brooks Koepka Island-Hopping with Hot Model GF in Thailand

U.S. Open Champ Brooks Koepka and his hot model GF, Jena Sims, ain't ready to let the summer go -- bustin' out the bikinis and boardshorts for a red hot Thailand adventure!!

It's been almost 5 months since Brooks won the big one ... but him and Jena are still living the good life -- hangin' with monkeys, backflippin' off boats and even getting in some ass-cuppin' PDA as they cruise around the Phi Phi Islands with their buds.

Jena's a former Miss Georgia Teen USA, FYI -- and it looks like Brooks has some local competition for his beauty queen ...