The romance between the 'Milk' actor and Shannon Costello is said to have blossomed when they both worked together for his non-profit J/P Haitian Relief Organization charity, where she was formerly communications director, according to People magazine.
Sean, 51, and Shannon, 26, are said to have been friends since 2009 and were first spotted together in July after Sean's three-month relationship with Scarlett Johansson had ended.
They have been pictured on the beach together in Malibu, California, and dining at Nobu and out together in Miami.
Shannon is currently co-producing Ron Howard's upcoming film, 'Under The Banner of Heaven'.
Sean split from Scarlett – who was formerly married to Canadian actor Ryan Reynolds - in June as he wasn't ready for a "serious" relationship.
Sean's 14-year marriage to Robin Wright was officially ended in July 2010, and he has two children, daughter Dylan and son Hopper, with her.