Chris Brown -- Karrueche Didn't Dump Him ... He Dumped Her.

Chris Brown -- NOT KARRUECHE -- is the one who did the breaking up, because she was a crass opportunist ... so say people in Chris' camp.

Sources closely connected with the singer tell TMZ ... Chris is shocked Karrueche's people leaked a story that she dumped him, allegedly because of a never-ending obsession with Rihanna and refusal to act like an adult.

But Chris' people tell us ... he found out she was texting and sending photos to other guys while they were together. He was also furious she did interviews for her clothing line and talked about their relationship.

In a word ... Chris decided she was an "opportunist" and dumped her.