After Justin threatened to make his Instagram private when fans hit out at his reported new girlfriend Sofia Richie, 17, Selena offered him some advice.
She wrote on Instagram: "If you can't handle the hate then stop posting pictures of your girlfriend lol - it should be special between you two only. Don't be mad at your fans. They love you (sic)."
Justin was not happy and replied: "It's funny to see people that used me for attention and still try to point the finger this way. Sad. All love. I'm not one for anyone receiving hate. Hope u all can be kind to my friends and each other. And yes I love my beliebers (sic)."
However, Selena, 24, then hit back and accused Justin of cheating her throughout their on/off relationship which ended in 2014.
She wrote: "Funny how the ones that cheated multiple times, are pointing the finger at the ones that were forgiving and supportive, no wonder fans are mad. Sad. All love (sic)."
And then Justin, 22, reportedly slammed Selena by saying: "I cheated...Oh and I forgot about you and Zayn (sic)."
Selena previously admitted she would love to date Zayn, who is currently in a relationship with Gigi Hadid.
She said: "If he had asked me out on a date, I would be seen with him. Just kidding, but not kidding."
And she also praised the former One Direction star for his "clever" solo music.
Speaking before his album was released, she said: "I'm as excited as you to listen [to his album 'Mind of Mine']. Honestly, I was very surprised when the first single came out. I loved when I heard.
"I love the new songs on it. I think this new vibe and where it is going is quite clever."