Jacob Benjamin Gyllenhaal is an American actor. Born into the Gyllenhaal family, he is the son of director Stephen Gyllenhaal and screenwriter Naomi Foner, and the younger brother of actress Maggie Gyllenhaal. He began acting as a child, making his acting debut in City Slickers (1991), followed by roles in his father's films A Dangerous Woman (1993) and Homegrown (1998). His breakthrough roles were as Homer Hickam in October Sky (1999) and as a psychologically troubled teenager in Donnie Darko (2001).

The Gyllenhaal family is a Swedish noble family descended from cavalry officer Lieutenant Nils Gunnarsson Haal, ennobled in 1652 with a change of name to "Gyllenhaal".