The singer - who has previously denied she is back with Chris - posted the snap of her ex-boyfriend half naked after they were seen spending Thanksgiving together in Berlin.
While his face wasn't visible, his trademark tattoos were on show - as well as his boxers - as RiRi tweeted: "Dis n***a....... #BartObsessed".
Rihanna - who later jetted to London to perform on last night's 'X Factor' - had been in Germany to see Chris perform as part of his Carpe Diem tour, just a day after finishing her own 777 tour.
However, in a recent interview Chris said that he and Rihanna were just "working on their friendship."
He told US radio station Power 106: "Me and her have history, and me and her are definitely always going to be best of friends."
His tweets to Jenny Johnson included: "Take them teeth out when u Sucking my d*** HOE" and "mom says hello... She told me not to s**** in ur mouth, wanted me to s*** right on the retina".
What a charmer.