Foster was born in Boston, Massachusetts, the son of restaurant owners Gillian and Steven Foster.
He has an older brother, Ben, who is also an actor, and who has described their parents as "free-spirited, Vietnam-protesting hippies".
His paternal grandparents were Celia (Segal) and A. (Abraham) Frank Foster, a prominent judge and politician in Boston; their families were Jewish immigrants from the Russian Empire.

 His family relocated to Fairfield, Iowa after their Boston home was burgled while they were present.
Foster was born in Boston, Massachusetts, the son of restaurant owners Gillian and Steven Foster. He has an older brother, Ben, who is also an actor, and who has described their parents as "free-spirited, Vietnam-protesting hippies". His paternal grandparents were Celia (Segal) and A. (Abraham) Frank Foster, a prominent judge and politician in Boston; their families were Jewish immigrants from the Russian Empire.
 His family relocated to Fairfield, Iowa after their Boston home was burgled while they were present.
Jon Foster