If James Harden was bummed about not being in the NBA Finals ... he ain't showin' it -- 'cause he was TURNING UP with shirtless Travis Scott onstage Saturday night!!
Just five days after being bounced from the playoffs ... the Rockets PG turned to his ex-GF's sister's BF to get away from it all ... and he got away from it.
The Beard joined Kylie Jenner's baby daddy during the last few minutes of his Governors Ball set, rockin' out to "Beibs in the Trap" ... and the crowd loved it -- giving a "F*CK THE WARRIORS" chant afterward.
Remember, Scott's a huge Houston fan ... and was spotted at Harden's games against the Dubs late last month.
Sunday night had to be tough watching Golden State go up 2-0 on the Cavs ... but hey, at least Saturday night was fun!