The 23-year-old American singer and 26-year-old Australian actor were spotted together at the event but did not walk the red carpet. Hemsworth's brother Chris Hemsworth stars in the Snow White and the Huntsman prequel.
"I watched [Miley and Liam] both run to the restroom. They were kind of giggling because they were both running," an eyewitness told E! News. "They were really nice when I asked them to take photos with them."
Cyrus and Hemsworth's outing follows word the couple's engagement is back on. The pair reportedly reconciled and got engaged again over the holidays after calling off their first engagement in 2013.
"It's great [having Miley back in the fold]," Chris Hemsworth said at the premiere. "It's good getting together with everybody. We don't see much of each other. We're across the globe -- I'm back in Australia and Liam's here."
Cyrus, who was spotted wearing her engagement ring in March, was seen at lunch with Hemsworth and his brothers Friday in Los Angeles. A source told People magazine the couple had just spent a few days together in Malibu, Calif.
"Miley has been going back and forth between L.A. and N.Y.C. for the Woody Allen series," the insider said. "She was happy to spend some time with Liam -- things are great between them. There are no set wedding plans, and they are enjoying their engagement."
Cyrus will star with Allen in a new comedy series for Amazon. She will also serve as a coach on The Voice Season 11, while Hemsworth is slated for Independence Day: Resurgence with Maika Monroe.