Kourtney Kardashian -- Miami Run-In ... With Bieber

Kourtney Kardashian and Justin Bieber re-ignited the buzz Sunday in Miami ... that they're FWB's.

Kourtney was in town to celebrate the birthday of club honcho Dave Grutman, and Biebs happened to be in Miami for his show.

Kourtney and her crew went to STORY nightclub, and Biebs showed up after the concert, sang happy birthday to Dave and then did his thing at the DJ booth. He pulled Kourtney from her table up to the booth where they spent some time.

BTW -- Kourtney says she never hooked up with JB.