According to the photographer, the "Birthday Cake" singer got to Ashton's home right around midnight ... and didn't leave 'til roughly 4 AM.
As we previously reported, Justin Bieber recently checked out Ashton's $10 million bachelor pad on his high-end real estate hunt ... but we're guessin' Rihanna wasn't there for an open house.
So far, no comment from Rihanna's rep.