"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, quote, "They had another huge fight, and Selena won't forgive him...{She's} done with him." (--Did it have anything to do with Justin's new "green" habit? He was photographed toking just a few days later.)