The R&B superstars were spotted making out at Griffin nightclub Monday night, reported Us magazine.
The former couple were "hugging and kissing," an eyewitness said.
Earlier this year, multiple sources claimed that Brown, 23, and Rihanna, 24, have been secretly hooking up.
The on-and-off twosome split in February 2009 following Brown's brutal assault of the "We Found Love" singer.
At the 2012 MTV Video Music Awards on September 6, Brown publicly congratulated Rihanna on her Video of the Year win by kissing her on the lips.
Brown's girlfriend of nearly a year, aspiring model Karrueche Tran, 23, was "totally pissed off. She felt the kiss was a public slap in the face," a source said.
On Monday, Tran tweeted, "There's a difference between a man and a boy. I prefer men."
