The 24-year-old British singer and 21-year-old American actress attended the Backstreet Boys' residency show Saturday at The Axis at Planet Hollywood in Las Vegas.
One fan captured Horan and Steinfeld singing and dancing along to several songs, including "I Want It That Way." Backstreet Boys singer AJ McLean came up to Horan at one point in the song so the star could sing along.
E! News reported Horan and Steinfeld visited XS nightclub after the concert, where they enjoyed cocktails and a DJ set by Diplo. The pair sat together at a stage table with friends.
Horan and Steinfeld were previously spotted together at the 2017 American Music Awards in November. The One Direction singer dedicated a sweet post to Steinfeld on her 21st birthday the next month.
"Happy birthday to the loveliest person on the planet and one of my best friends @haileesteinfeld," he wrote. "Love ya hails."
Steinfeld addressed her reported romance with Horan in an interview with Us Weekly in January, saying she appreciates her fans' interest but prefers to keep her personal life private.
"I feel so lucky that [my fans] care enough about me to know what I'm doing every second of the day, but I think that sometimes it's hard for people in general to realize that there are boundaries to a person's life and personal life," the star said.