The "Confident" singer also flaunted her super-fit bod in a sexy black one-piece swimsuit, looking flawless in the hard-to-pull-off style.
"Wilmer and Demi could not keep their hands off each other," a source tells ET. "They looked really happy and super affectionate."
ET confirmed on Wednesday that Demi is currently in St. Barts for the Victoria's Secret Swim Special airing in 2016, which her pals, Nick and Joe Jonas, are also appearing in. Demi took to Instagram to share photos of the three of them posing with the gorgeous models.
She also confirmed her boyfriend joined the trip, apologizing to Nick for "third wheeling" him. "I love these pics so much... Sorry @nickjonas for third wheeling you but your bromance third wheels me too!!! ? #somethingbigiscoming16 #CONFIDENT #myboys #baes," she wrote.
We doubt Nick minded too much. Wilmer and Nick are friendly with one another, and even smoked cigars together while the three hung out on the beach, our source tells us.