Justin Bieber's fling with Kourtney Kardashian is an "ego boost" for him.

The 21-year-old singer has reportedly been hooking up with the 36-year-old mother-of-three and is thrilled that she is interested in him.

Woman's Day revealed that the stars were seen together a few weeks ago at Nice Guy, a restaurant in LA, before sneaking away to Bieber's Montage Beverly Hills hotel room, where Kardashian was spotted leaving around 4am.

A source told the gossip magazine: "She's calling it a fling, but her feelings are clearly stronger than she's letting on.

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"She's constantly texting and wanting to see him. She's been secretly smitten with him for months."

A source also told People that "(Bieber) has had a huge ego boost from hooking up with Kourt. It's been going on for a few months. He is used to being swooned by young girls that have nothing else going on. Kourt is a hot mom with a career."

Kardashian's former partner Scott Disick, 32, the father of her three children, is said to be furious at the fling and believes she is only using Justin to get back at him but the Sorry hitmaker doesn't care.

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The insider said: "He more thought, 'Wow,' and couldn't believe that she would go for him. He brags about how hot she is. He is happy while it lasts. Even if she is using him to get back at Scott, he couldn't care less.

"He's obviously close to the family, and Kris [Jenner] is obsessed with him. It's just one of those things where it makes Kourtney feel good about herself. It's not serious at all. This young guy is into her, he thinks she's hot and she's older. She just wants to have fun."

Bieber added fuel to the rumours after posting a photo to Instagram of him with a mysterious woman leaning up against a car, thought to be Kardashian. He captioned the picture "Lord knows".

Lord knows

A post shared by Justin Bieber (@justinbieber) on

- Bang! Showbiz