A source tells People magazine that Theron traveled to the island with her 2-yaer-old son Jackson to celebrate the holidays at Penn's beachfront home in Oahu.
The Academy Award-winning actor, 53, joined them later and was spotted surfing the waves Tuesday afternoon as the 38-year-old actress tanned in a multicolored striped bikini.
The duo was spotted stepping off the same flight at LAX airport Wednesday and on Thursday they were seen leaving Theron's house in separate cars.
"Charlize and Sean have been friends for years," another source told the magazine. "They have been spotted out together several times in the past few years. Even when Charlize was dating Stuart [Townsend]. They both had dinner with Sean several times."
Rumors that the actors were dating began in 2010 after Theron split from Townsend.