The ex-lovers have been romantically linked since the beginning of the year and after they collaborated on remixes of each other's songs.
Brown is said to have flown on a private jet to Nice, and then a helicopter to Saint-Tropez, where he arrived on multi-million pound boat which has been chartered for a week.
He reportedly wanted to see Rihanna after he heard that she was joining love rival Drake for a holiday together.
"Chris is staying on the yacht next to Rihanna's. He's got rappers Swizz Beatz and Ludacris with him and they've decided to film a music video too," a source told The Sun.
"It was a last-minute trip, but Chris wanted to see Rihanna. They have this weird addiction to each other and can't seem to stay away.
"She was really happy when he arrived. She spent all evening getting ready to see him. They waited till night so no-one saw them.
"She went to his yacht, which they moved to a secluded bay. They've been through a lot, but still have strong feelings for each other."
Rihanna's father recently approved of his daughter's relationship with Chris Brown, describing them as "perfect" together.
The 'Where Have You Been' singer was believed to have been the cause of a scuffle between Brown and Drake in a New York nightclub last month.