Chris Brown I'm NOT Back with Rihanna

Chris Brown and Rihanna are NOT back together, despite rampant rumors to the contrary ... Breezy's rep tells TMZ.

As we previously reported, Chris and Rihanna were spotted out at the same West Hollywood nightclub Sunday night. They left separately, but there was lots of speculation that the two had gotten back together.

But that's not true -- according to Chris' rep, the singer was with his actual girlfriend last night ... an aspiring model named Karrueche Tran ... not with Rihanna.

The rep adds, Chris and RiRi are just friends.