"It's not easy," a source told PEOPLE, "but the relationship needed to end."
Clues that the young couple, who dated for nearly two years, were in trouble first surfaced this summer.
Bieber, 18, was late to a Fuse TV taping in June "because he and Selena were fighting," a different source told PEOPLE at the time, while another insider revealed that Gomez, 20, "was crying about Justin" during pal Ashley Tisdale's birthday party on July 2.
Although they quickly reconciled, things have seemed amiss in recent days.
Though Gomez was spotted at Bieber's Minneapolis concert on Oct. 20, a week later, he posted a photo of himself on Instagram with the caption, "Just me." Shortly thereafter, he posted a black-and-white photo of himself looking plaintive with the message "lingse" – which fans were quick to point out spelled "single" when the letters were unscrambled. He soon deleted it.
Moving on Already?While Gomez had been busy shooting a reunion special for her hit Disney show Wizards of Waverly Place in L.A., Bieber performed at the Victoria's Secret Fashion Show in NYC on Wednesday before hitting an afterparty at Lavo where some of the models were "making a fuss over him," an observer tells PEOPLE.
The following night, a witness saw Bieber and his security picking up two brunettes at NYC's Alex hotel. Bieber then went to see The Lion King on Broadway, where the two women were also photographed. One was identified by the New York Post as Hungarian model Barbara Palvin.
The model, 19, also posed with Bieber and pal rapper Lil' Twist in a shot the rapper posted online before taking it down – but not before Gomez posted the shot on her own Twitter feed with a set of ellipses.
(For her part, Palvin Tweeted, "hey everyone. please calm down. he is all yours!! :) please :)")
In an interview with PEOPLE in early September, Bieber's mother Pattie Mallette may have foreshadowed a split. Gomez "is a sweetheart," she said. "But I don't think there are plans for [marriage] in the near future."
Bieber himself seems unsure as to the current state of his relationship.
"I don't know what to say," Bieber told John Garabedian on his Open House Party radio show Saturday. "I don't know really what's even going on in my life, so ... to even assess that doesn't make sense 'cause I've not made any comment."
Reporting by JESSICA HERNDON, CARLOS GREER, KEVIN O'DONNELL and JULIE DAM