The late rapper would have been 27 on Saturday, January 19, and the "Thank U, Next" hitmaker shared a cryptic post on Twitter, which prompted her fans to speculate it was about Mac.
"Miss u," she wrote on Twitter, before quickly deleting the post.
View this post on Instagram
Grande publicly remembers Miller on his birthday yesterday! (Source: E! News) . . . . . . . #oneweeknews #macmiller #arianagrande #he #was #an #angel #remembering #birthday #public #twitter #missu #missyou #miss #you #singers #singer #rapper #rap #hiphop #exgf #ex #exbf #gf #bf
Hours later, Ariana shared a series of sweet photographs of the pair during their time together on her Instagram Story, including one in a recording studio, and one on a beach, but again, deleted the images.
The former couple dated for two years, before splitting in May last year (18).
Mac, real name Malcolm James McCormick, was found unresponsive in his California home on 7 September (18). The cause of death was due to an accidental overdose of fentanyl, cocaine and ethanol.
Ariana, 25, has shared numerous tributes to the "Self Care" hitmaker since his death, including in December when she posted - and deleted - a photo of the late rapper playing the piano.
Mac's mother, Karen Meyers, also shared a heartbreaking tribute to her son on his birthday.
She shared a picture of the original announcement that went out when he was born in 1992, according to The Blast.
"Our son Miller is very excited about the birth of his little brother Malcolm James, who was born at eight forty six a.m. on January nineteenth and weighed in at exactly eight pounds," read the announcement.
The front of the printed document featured a drawing of confectionery M&M's - a sweet reference to Mac's initials - and stated the future rapper is "a real sweetie."