The 24-year-old 'Diamonds' singer, who was causing controversy during her '777' world tour this week, attended Brown's afterparty following his concert at the O2 in the German capital.
The former couple spent the evening chatting closely, smoking cigarettes and enjoying shots until the small hours at Adagio nightclub
The Sun claims that the pair then left together and headed back to Brown's hotel.
Rihanna and Brown were recently seen kissing and cuddling at a club in Los Angeles, and Rihanna's father Roger Fenty recently backed their reported reunion, saying: "I know they love each other. They always have.
"She's happiest when she's around him, and as long as she's happy, I am happy and the whole world should be too."
Photo gallery - Rihanna's wild antics on Twitter: