Both Cyrus and Carter had split from their previous partners, Liam Hemsworth and Brody Jenner, respectively, earlier this summer.
“They’ve been friends forever and were there for each other when they were both getting separated,” the source said.
Earlier in August, Cyrus confirmed she and “Hunger Games” star Hemsworth, 29, had separated after less than a year of living as husband and wife. Carter and Jenner, 36, also broke up following an elaborate wedding ceremony, which took place last year. The two women were spotted together on numerous occasions over the past month, including their appearance at Soho House in West Hollywood, where they were seen “basically having sex.”
“They were obsessed with each other. They couldn’t keep their hands off each other,” a source previously told Page Six.
The couple also hit a private New York Fashion Week event earlier in September. “[Miley] was all touchy-feely with Kaitlynn the whole night and kept making out,” another source said. “During the night, they stood up and started dancing with their friends in the section they were in and kept making out against the wall.”
Reps for Cyrus have not commented on the breakup yet.