Miller, 26, died of a suspected drug overdose.
When he spoke with Rolling Stone earlier in the year, Mac said that he would never call himself a drug addict.
The pop star, who has remained silent since news of Miller's death broke Friday, posted a black-and-white photo of the rapper looking up at the camera Saturday afternoon.
The photo had no caption and comments were disabled. Both players took to Twitter to express their personal sense of loss over Miller's untimely passing.
The Self Care rapper, real name Malcolm James McCormick, was found unresponsive at his San Fernando Valley, Calif., home, and was pronounced dead at the scene by authorities.
Ariana addressed the breakup in an Instagram story, writing of Miller: 'I respect and adore [Malcolm] endlessly and am grateful to have him in my life in any form'.
She tweeted back: "I am not a babysitter or a mother and no woman should feel that they need to be".
In May, shortly after their breakup, Miller crashed his vehicle and was charged with driving under the influence.
"Let's please stop doing that", she continued.
Sources have claimed she has been left devastated by the news.
"Of course, I didn't share about how hard or scary it was while it was happening but it was", she wrote in May.
Miller has been open in the past about his battles with substance addiction.
"I will continue to pray from the bottom of my heart that he figures it all out and that any other woman in this position does as well".
"Although the two ended their relationship, Ariana always shared a deep connection with Mac and wanted nothing but the best for him", the source said.