Sources close to the deal tell TMZ ... Rihanna recently purchased the crib above the Sunset Strip for a cool $6.8 million. It's got 6 bedrooms, 9 bathrooms and just over 7,000 square feet of space. There's also a movie theater inside, a fully equipped gym, a fireplace, a billiard room and a guest house ... plus a pool and a huge yard.

It's also gated ... which could come in handy around those parts. As we reported, Kendall recently unloaded her WeHo home after about a year of grief -- which included a stalker encounter and a high dollar theft.