Chelsea Handler is literally sliding into her 50th bday ... while skiing the slopes and showing off her amazing bod in a sexy red bikini!

The comedian -- who's never at a loss for words -- is looking fitter than ever as she zooms down a snowy summit with a couple of ski poles to help her navigate some of those tight turns to ring in The Big 5-0.

And Chelsea couldn't be happier ... she's grinning from ear to ear in this series of hot pics on the cold mountain peaks.

Chelsea is also taking total charge ... Look at her leading a pack of in-shape, bikini-clad women on skis as they follow Chelsea to the bottom of the mountain.

And the fun doesn't stop there ... Chelsea rides on a snowmobile, smokes a joint, and plays with a furry black dog.

Ruff! Happy 50th, Chelsea!!!