Michael Phelps -- Jingle Balls

No pants? No problem.

Michael Phelps -- who just finished up a stint at a rehab center -- brought a 6-pack to Christmas.

The Olympic swimmer and his girlfriend Nicole Johnson (Miss California 2010) spent the holiday together ... along with some friends and posed for a hilarious pic.

Phelps posted the pic on Instagram and wrote, "Yep.... I forgot my pjs for the pjs party .... Oops."