Bruce Willis and Demi Moore's youngest daughter posted a bikini hot shot on Christmas Eve alongside her 2 sisters, Rumer and Scout -- who were also bikini-clad -- as they all took a dip in what looks like a river surrounded by snow.

Tallulah captioned the photo ... "dedicated to everyone who called me ugly at 13."

The youngest Willis kid has made it known she used to be bullied about her looks growing up. She told Katie Couric in 2014 ... "I had people when I was 13 telling me how ugly I was, and how could two beautiful actors create such hideous troll children?"