Josh Hartnett was taken to Cedars-Sinai Medical Center by ambulance at 0145 local time on Monday morning.
Susan Patricola said in a statement the star was “under observation but is resting comfortably”.
Hartnett made his West End debut in a stage adaptation of the Oscar-winning film Rain Man last year.
Ms Patricola said the 30 Days of Night star was “suffering from a flare-up of a gastrointestinal problem that plagued him while he was starring in the West End during the production of Rain Man”.
The actor was taken ill for a few days with the same problem last November while in London.
Hartnett is currently in the middle of production for his next film, Queen of the South, with Sir Ben Kingsley.