The reality star was linked to Rihanna after the duo were photographed go-karting and partying together in Los Angeles.
However, Kardashian has now told Ryan Seacrest on his KIIS-FM radio show that he is just friends with the 'Where Have You Been' singer.
"Rihanna's a friend, and she is friends with my friend Capricorn and it was Cap's birthday," he explained. "It was a little birthday thing. And then we all went out to celebrate her birthday. It was fun, it was a good night."
Kardashian also denied persistent rumours that he is dating the British singer Rita Ora, who has also laughed off the reports.
"Rita is in the UK, I'm just a fan," he said. "I'm a big fan... She will be in town for the VMAs.
"I'm a fan, I'm a fan. Seriously, though, I wish you could buy UK albums here."
Listen to Rob Kardashian speaking to Ryan Seacrest below: