Justin Bieber is heating up his winter vacay by stripping down to just his Calvin Klein briefs ... and giving fans a close-up look at what he's packing.

The pop star posted on his Instagram Stories Thursday evening, sharing a series of pics from his winter getaway ... including a couple where he sported nothing -- and we mean nothing -- but the designer underwear.

Check it out ... JB's lounging in a wooden recliner, catching some rays next to a brisk brook and quite a bit of snow. Justin appeared to be a bit damp, hinting the "Yummy" singer may've participated in a "Polar Bear Plunge" of sorts ... a dip in a freezing-cold body of water in the middle of winter.

Regardless of the reason, Justin transmitted a very important message with these pics... nothing comes between him and his Calvins. (The hitmaker has been a longtime ambassador for Calvin Klein, for those wondering.)

While Justin didn't reveal where he was off vacationing, it's clear he enjoyed the cold weather at a ski resort ... the singer also uploaded a photo of himself -- this time bundled up -- sitting on a snow mound, smoking in front of some skis.

Looks like Justin is focusing on rest and relaxation in 2025 ... after his 2024 was plagued with breakup rumors and alleged money mismanagement.