According to a new report from People, the mother-of-three has been "hooking up on and off [with Justin] for a few months now."
The bombshell revelations come as the reality star spends more time with her ex-partner and father of her children, Scott Disick.
Meanwhile, insiders tell People the unlikely duo first started seeing one another late last year and their most recent dalliance was during the LA stint of his Purpose world tour in March.
"They have hooked up a few times, including recently. It happens in L.A," the source revealed.
The Sorry hitmaker has even spent time with her kids Mason, six, Penelope, three, and Reign, one, who she shares with Scott.
"He has met her kids, but they don't hook up around the kids. It's on nights when she has help and is able to go out," the pal said.
Watch Kourt DENY her romance with Biebs in the player below. Post continues after the video!
However the insider adds that it's very much a casual set-up.
"Kourtney is great. Justin thinks it's cool that she is older and hot. She isn't clingy and has her own life," the source said.
Adding: "There are no bad consequences for Justin to hook up with her. She is very easy going and just makes it fun for him to be around her.”
In January during an appearance on Ellen, the 36-year-old was quizzed about her reported romance with Justin, 22.
“We’re friends,” she explained, before breaking out into a smile.
"You're smiling as if that maybe it's beyond friends," Ellen teased.
"I've been smiling this whole time since I've been sitting here," the LA local giggled.
Before Ellen quipped: “Does Justin Bieber help with the kids?”
“No!” she quickly responded.
This year alone Justin has been linked to model Hailey Baldwin and his ex-girlfriend, Selena Gomez.