Both couples were attending a VMAs after-party.
Kaitlynn Carter had a run-in with her ex after the MTV Video Music Awards.
Following the big event in New Jersey on Monday, Miley Cyrus, 26, and Carter, 30, were spotted walking hand in hand into New York City nightclub Up&Down around 1 a.m.
The pair -- who were recently seen kissing in Italy following Carter's split from Brody Jenner and Cyrus' breakup from husband Liam Hemsworth -- were there for Columbia Records' VMAs party, following Cyrus' emotional performance of "Slide Away" at the annual awards show.
Carter opted for a white T-shirt, gray coat, sneakers and jeans for the occasion, while Cyrus also kept it low-key, sporting a white crop top, a patterned jacket, a brown hat and ripped jeans.
An eyewitness tells ET that Cyrus looked "very happy and playful" with Carter at the party, adding that the women "didn't stay long."
About 20 minutes after Carter and Cyrus made their entrance, onlookers say Jenner, 36, showed up to the same club with his new flame, Josie Canseco, 22. The pair held hands as they entered the venue, with Jenner in an all-black look while his date coordinated with black jeans and a gray crop top.
The two couples stayed for about an hour, before Cyrus and Carter went back to their hotel. Meanwhile, Jenner and Canseco went on to party at another NYC club, Avenue.
The foursome's run-in comes after an eyewitness spotted Carter and Cyrus sharing a few sweet moments backstage before the singer took the VMAs stage. The women were seen holding hands and at one point Carter affectionately placed her hand on Cyrus' butt.
"They looked very much like a couple," the eyewitness told ET. "Kaitlynn was swaying at the side of the stage as Miley rehearsed 'Slide Away.'"
Following the performance "the two walked backstage, hugging each other," the eyewitness said.
Watch the video below for more on Cyrus' VMAs performance.