The 27-year-old Australian actor was spotted with Cyrus after the 24-year-old American singer delivered an emotional rendition of her song "Malibu" at the awards show in Las Vegas.
Cyrus, who was introduced by dad Billy Cyrus and sister Noah Cyrus, broke down in tears toward the end of her performance. She previously told Billboard that Hemsworth inspired "Malibu" and influenced her new album.
"They're going to talk about me if I come out of a restaurant with Liam. So why not put the power back in my relationship and say, 'This is how I feel,'" the star said in May.
Cyrus and Hemsworth reconciled in January 2016 after ending their engagement in 2013.
"I needed to change so much," the singer explained in the interview. "And changing with someone else not changing like that is too hard. Suddenly you're like, 'I don't recognize you anymore.' We had to refall for each other."
Drake was among the big winners at the Billboard Music Awards -- taking home 13 awards, including Top Male Artist. Cher was honored with the Icon Award and performed her hit song "Believe" at the ceremony.