The former couple were spotted kissing on the dancefloor at New York City hotspot Griffin before moving to the back of the club to get some privacy.
An onlooker told Celebuzz that Chris made a beeline for his ex-girlfriend, who was sitting at a neighbouring table, and "climbed over the centre to get to her."
The R&B singer was 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".
Further reports state that the pair were locked in a bathroom together for 30 minutes while two bodyguards guarded the door.
A friend of Rihanna`s told Hollywood Life: "It was on and cracking. They hooked up. I wasn`t in the bathroom with them but I know they f****d around and what not."
"They were in the club partying hard, had some drinks and kush and one thing led to another and that was that... He wanted her just as much as she wanted him."
Chris - who was charged with assaulting the pop star in 2009 - has made no secret of his close relationship with the `Diamonds` singer, and the pair even shared a public peck on the lips at the MTV Video Music Awards last month.
Eyewitnesses have taken to twitter to comment on the couple`s reconciliation, with fellow club-goer @kandibecool posting: "Btw, Chris Brown and Rihanna are def together or at least hooking up... Saw it 1st hand (sic)".
Chris` girlfriend Karrueche Tran appears to have heard the rumours, after she tweeted this morning: "there`s a difference between a man and a boy. I prefer men (sic)."
When a friend told her to "Let it go for your own good", the model replied: "oh baby I am."