The former couple were reportedly spotted "hugging and kissing" at New York City venue Griffin.
[Rihanna and Chris Brown: [L] Embracing at the MTV VMAs in September 2012; [R] The former couple on stage together in 2008]
An onlooker told Celebuzz that Brown approached his ex-girlfriend, who was sitting at a neighbouring table, and "climbed over the centre to get to her".
Brown was apparently seen "dancing on the table for Rihanna" and the two appeared "very smitten" as they were "hugging and kissing, and they went to the back of the club alone together".
A representative for Griffin confirmed that both Rihanna and Brown were at the club on Monday evening.
Further reports allege that the pair were locked in a bathroom together for 30 minutes while two bodyguards guarded the door.
Another source is quoted as telling Hollywood Life: "It was on and cracking. They hooked up. I wasn't in the bathroom with them, but I know they f***ed around and whatnot.
"They were in the club partying hard... one thing led to another and that was that... he wanted her just as much as she wanted him."
Brown was sentenced to five years' probation and 180 days of community labour for assaulting Rihanna in 2009.
However, the pair have reportedly kept a close friendship, and appeared to share a kiss at the MTV Video Music Awards in September.
Brown's girlfriend Karrueche Tran tweeted on Tuesday morning (October 3): "there's a difference between a man and a boy. I prefer men (sic)."
When a Twitter follower told her to "Let it go for your own good", the model replied: "oh baby I am."
Listen to Rihanna's new single 'Diamonds' below: