Rihanna & Chris Brown -- The Wrong One Goes Topless

They're like Justin and Selena ... but with protective orders. Chris Brown and Rihanna were on again ... off again ... and now apparently on again.

RiRi just tweeted this pic Saturday night of CB kickin' it half-naked in bed -- bare-chested with his drawers out -- then posted it for her 26 million Twitter followers.

It's the latest in their never-ending relationship saga ... as TMZ previously reported the two have been spending more and more time together in public, including Thanksgiving -- where they partied it up at the same German nightclub.

So we ask ...