The 42-year-old actor and 32-year-old actress went public with their relationship Sunday at the 2017 Cannes International Film Festival closing ceremony in Cannes, France.
Phoenix was spotted resting his hand on Mara's leg at the event, and embraced the actress before accepting Best Actor for his role in You Were Never Really Here. The pair were seen holding hands as they left the ceremony.
"They were talking to one another nonstop," a source told People. "Then they went for their car and he's holding her hand like it was unconscious. Like it fits. They seem a totally natural and easy couple."
Phoenix and Mara co-starred in the 2013 film Her, and will play Jesus and Mary Magdalene in the forthcoming movie Mary Magdalene. Sources told Page Six in January that the pair "found love" on the project's set.
"They are and have been good friends for quite a time," Phoenix's rep said in response.
Phoenix and Mara will also star in the new film Don't Worry, He Won't Get Far on Foot. The actor previously dated actress Liv Tyler and DJ Allie Teilz, while Mara was last linked to director Charlie McDowell.