The couple were first linked early last month when pictures of them kissing on a yacht in Italy surfaced only hours after Cyrus' split with her husband, Australian actor Liam Hemsworth, was confirmed.
Only a week prior to that, Carter and her ex, reality star Brody Jenner, were reported to be separating after a year of marriage.
It comes after one of Carter's co-stars claimed she had "a thing" with Cyrus last year before Hemsworth and Cyrus wed in December.
In the most recent episode of The Hills: New Beginnings, Carter is upset that her relationship with Jenner was rumoured to be "open", which she said was no one's business.
"It's tough for me to listen to Kaitlynn complain about people talking about her allegedly open relationship with Brody when we know she had a thing going with Miley. She already had something going on with her when these scenes were filming — maybe not physically, but definitely emotionally," reality star Spencer Pratt told Cosmopolitan.
Cyrus has denied being unfaithful during her relationship with Hemsworth.
In the episode, Carter tells her friends she wishes people had "common decency" when discussing other people's private lives, which Pratt claimed was hypocritical.
"I would love to know Liam Hemsworth's thoughts on Kaitlynn's recent displays of 'common decency'," he told the publication.
"You know what I call common decency? Getting it on at your Italian villa and not in front of the paparazzi."
Cyrus and Hemsworth both took to Instagram not long after news broke to address their break-up.
Hemsworth, who has been in Byron Bay as he deals with the fallout, said: "Just a quick note to say that Miley and I have recently separated and I wish her nothing but health and happiness going forward."
Meanwhile, Cyrus had a little more to say on the matter after she was accused of cheating.
"Once Liam and I reconciled, I meant it, and I was committed," she wrote.
"There are NO secrets to uncover here. I can admit to a lot of things but I refuse to admit that my marriage ended because of cheating.
"Liam and I have been together for a decade. I've said it before & it remains true, I love Liam and always will. BUT at this point I had to make a healthy decision for myself to leave a previous life behind."
