Rihanna's locking it down -- she and Drake are officially exclusive.
Multiple sources extremely close to the couple tell us, Rihanna and Drake have gone from casually hooking up to seriously dating ... they've decided to give it a serious shot.
We're told Rihanna wants to spend as much time with Drake as her schedule will allow -- because he treats her better than anyone she's ever been with. Not a high bar.
As we reported, the pair have been virtually inseparable while Drake's been touring in Europe -- we're told Rihanna has spent every single night with him since she arrived.
As for Drake, he's smitten -- one source said "he's in the best mood he's been in a long time."