Felton, who played Draco Malfoy in the Harry Potter movies, supported Radcliffe at the actor's Broadway play.
Felton shared a photo with Radcliffe after seeing The Lifespan of a Fact at Studio 54 Theatre. He praised his former co-star in the caption.
"Go see it," the star wrote. "He's fantastic as per @lifespanofafact."
Felton was in New York to promote the YouTube series Origin at New York Comic-Con. The show co-stars Natalia Tena, who played Nymphadora Tonks in the Harry Potter films.
Felton and Radcliffe co-starred in all eight Harry Potter movies, which concluded with Harry Potter and the Deathly Hallows - Part 2 in 2011. Radcliffe told The Independent in 2016 that he is "incredibly proud" of the series.
"I don't want to people to forget about Harry Potter. I'm incredibly proud of those films," the actor said.