Before leaving Instagram, Bieber, however, responded to Gomez's allegations, writing, "I cheated.. Oh I forgot about You and Zayn?" The comment was later deleted.
The exes began arguing on August 14 after Bieber, 22, threatened to make his page private if his fans continued to post mean comments about his rumored new flame Sofia Richie.
Responding to this, Gomez, 24, asked him to stop posting photos of the 17-year-old model and stop blaming his fans.
Later, Bieber appeared to diss her as he commented on his own post, "It's funny to see people that used me for attention and still try to point the finger this way."
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In another comment, the "Hands to Myself" singer then revealed that her ex-boyfriend cheated on her "multiple times" when they were still dating.
"Funny how the ones that cheated multiple times, are pointing the finger at the ones that were forgiving and supportive," she wrote. "No wonder fans are mad. Sad. All love."
Bieber and Gomez dated on and off for years before splitting in 2014. This feud is particularly upsetting for Gomez-Bieber shippers, who hoped to see them get back together.