Scott Disick "doesn't like" seeing Kourtney Kardashian with other guys.
The former couple - who have children Mason, 11, Penelope, eight, and six-year-old Reign together - went their separate ways in 2015 but the 37-year-old star admitted it still "bothers" him when he sees his ex-partner get "flirty" with other men.
In a preview clip from an episode of 'Keeping Up With the Kardashians', the pair are sitting by a pool when Scott says: "I feel like it annoys me when you flirt with this lifeguard."
Though Kourtney insisted she wasn't flirting with the lifeguard, Scott admitted it still upset him.
He told her: "Maybe it's just, in my head, seeing you around any guy bothers me... Maybe it's just something, I don't know, I feel like you're a little flirty."
He went on to admit that seeing Kourtney with anyone else is his "insecurity".
He added: "I just don't like seeing you with another guy.
"It hurt me when you were with somebody else," he says. "And, like, waking up to looking at pictures of you with this guy you were dating. You know, it was just unhealthy. It just made me upset and sad, you know what I mean?"
Though Scott insisted he has "no real problems" now and is more "carefree" than in previous points in his life, he has a "big fear" that their close friendship will change again if they embark on new relationships.
He told her: "The big fear is if you start dating again, then it goes back to that and I have to feel a different way again.
"It's just hard. It sucks. And I guess now that you're single and I'm single and you haven't been with somebody in a bit, either we figure out creating our own lives together or separately."
And in a confessional, Scott admitted the "limbo state" he and the 42-year-old Poosh founder have found themselves in "isn't that cool anymore".
He said: "It's definitely difficult that Kourtney and I basically spend 90 percent of our lives together, travel together, do everything together, see each other every day, and, you know, the only difference is, at the end of the night, we kind of part ways and go and sleep in separate houses.
"This whole limbo state isn't that cool anymore. I surely don't want us both to wake up and be 50 years old and doing the same thing."
Since the clip was filmed, Scott is now dating Amelia Hamlin, while Kourtney has found love with Blink-182 rocker Travis Barker.