'Bachelorette' Runner-Up Nick Viall -- Blows His Loser Cover Before Show Finale (PHOTO)

'Bachelorette' runner-up Nick Viall ruined the big finale -- making it clear he wasn't picked -- because he was openly and notoriously cavorting with a hot girl who was NOT the bachelorette.

Viall was hanging poolside Sunday afternoon at the Mondrian Hotel in West Hollywood, clearly not caring that everyone around him could easily guess he was not bachelorette Kaitlyn Bristowe's choice. It's all the more interesting because most people assumed he was gonna win during Monday's show.

We're told Nick is saying the chick in the pool is just a friend from home.

He clearly has a type ... she looks just like Kaitlyn, but we're not calling her a consolation prize.