The duo came in separate groups to nightclub Griffin in the early hours of Tuesday morning, and got into the bathroom after dancing and getting cosy on the dance floor.
Brown arrived with friends, including rapper Bow Wow and sat at a table, and five minutes later, Rihanna, who had been partying with friends at Catch, walked in with her friends, reports nypost.com.
"They sat two tables away from each other. Chris made his way over to Rihanna. He raised his shirt and was dancing promiscuously. Then they started dancing together and hugging and kissing in front of everyone," said a source.
If observers are to be believed, they were then seen going into the bathroom together, and then when Rihanna emerged, "she seemed a little ruffled", said the source.
Rihanna broke up with Brown after he hit her during a fight before the 2009 Grammy awards.
At Griffin, they are said to have partied together until 4 a.m. However, a representative of the nightclub declined any comment.