"Us Weekly" says JUSTIN BIEBER and SELENA GOMEZ have broken up, and it's for good this time.  They had flown to Mexico for a New Year's vacation, but Selena bailed on December 30th, and they both ended up spending New Year's in the States, separately.

A source says, "They had another huge fight, and Selena won't forgive him, [She's] done with him."