The source says Kourtney flirted and Justin decided to make a move, and they've hooked up a couple of times since then ... the last being this past weekend when Kourtney left Justin's Beverly Hills hotel at 4 AM.
Her family is reportedly asking her questions like cray, but she's insisted all she's done with Bieber is hang out with groups of people in the hotel.
Justin and his peeps beg to differ ... as one source says, "He's single, she's single and they're just having a good time. Nothing serious."
We're pretty sure Justin's hooked up with Kourt's sister Kendall Jenner, too. #Gross?