Robert Pattinson & Kristen Stewart -- DATE NIGHT ... at the Prince Concert

The couple formerly known as Kristen Stewart and Robert Pattinson ... reunited last night to see the artist formerly known as Prince ... in what appeared to be a real deal date night ... and TMZ has the pic.

Rob and Kristen rolled up to the Sayers Club in Hollywood in the same SUV ... and sneaked into the back entrance to see the Purple One live in concert.

A short time later, the couple rushed back into the car -- trying to cover their faces -- but it was too late ... they'd been made.

It's unclear if the two are officially back together -- or if they're just toying with the media in what could be a shameless publicity trick to hype up their new movie.

In related news, the show was pretty good -- he played "JUNGLE LOVE" .... the one he wrote for Morris Day & The Time!!!!!!!