The singers performed separately on Sunday's Grammy Awards show.
Brown has skipped the Grammys ceremonies for the last few years. He was arrested for assaulting his then-girlfriend Rihanna during a heated argument the day of the 2009 prize presentation in Los Angeles. Both he and Rihanna were scheduled to perform at the show, but they backed out at the last minute.
Brown later pleaded guilty and was sentenced to probation and community service, and was ordered to attend anger-management classes.
Rihanna released a full-length version of her song "Birthday Cake" featuring new verses by Brown Monday, her 24th birthday.
Brown also tweeted a link to the new version of his song "Turn Up the Music," featuring additional music by Rihanna, which Rihanna later re-tweeted, UsMagazine.com said.
Brown, 23, is currently living with aspiring model Karrueche Tran.
But UsMagazine.com quoted multiple unnamed sources as saying Brown and Rihanna have been secretly meeting to have sex for the past year.