Kylie Jenner 's making Santa's signature color sexy this holiday season ... flashing her ample cleavage in a tight and tiny leather jacket.

The reality star shared pics and videos of her ensemble to social media ... taking mirror selfies and showing off her top, which clung to her chest and didn't quite meet her pants at the waist. Gallery Launch Gallery Instagram / @kyliejenner

Jenner shared a Boomerang ... capturing a small lean forward and back which accentuated her curves -- before stepping away from the camera in the final clip to show off the full 'fit, black pants included.

Kylie played around with her hair, too ... sometimes letting it drape down as a curtain for her chest -- while throwing it back at other points to give fans a full glimpse of everything she's got work

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