The 23-year-old singer and upcoming "The Voice" coach was spotted wearing the 3.5 carat diamond stunner as she left the Soho House with Hemsworth on Tuesday.
She and the 26-year-old actor held hands as they exited the members-only club.
The sighting comes amid rumors Cyrus and Hemworth's engagement is back on.
The pair called off their engagement in 2013 after four years of dating, but reportedly reconciled over the holidays.
Hemsworth denied he is engaged in April, but later appeared to confirm the news in the June/July issue of GQ Australia.
The actor played coy about his status with Cyrus in the interview, saying it should be obvious to fans.
"People will figure it out, they already have... They're not dumb," he told the magazine.
"Of course [the split] was hard," the star admitted. "But at the time we were going in different directions and it's just what needed to happen. We were both super young and it was a good decision at the time -- we both needed that."
Cyrus' dad, Billy Ray Cyrus, told the "Today" show the couple are "really happy together right now" during an appearance this month.
The 54-year-old singer also offered to officiate the wedding should his daughter and Hemsworth marry.
Copyright 2016 United Press International, Inc. (UPI). Any reproduction, republication, redistribution and/or modification of any UPI content is expressly prohibited without UPI's prior written consent.