Paris Hilton Kissing A Man On The Beach -- Hey, That's Not Her Boyfriend!

Paris Hilton and boyfriend River Viiperi are on the outs, cause she was sucking face with some guy on the beach this weekend ... someone other than River.

Hilton and her new male model friend Josh Upshaw were in Malibu Sunday when they got deep into a make out sesh ... right in the open. According to our sources, Paris and River ended their relationship amicably recently.

Neither has spoken about the break-up, but they haven't been seen together for several months now.

The River no longer runs through it.