According to Britain’s The Sun, Miller, 25, and Carter, 38, are secretly a couple. The two shared a love scene in the film Factory Girl, in which the young actress portrayed Edie Sedgwick, the muse of Andy Warhol.
A source revealed, "Sienna is naturally very flirty and couldn’t keep her hands off Will. They seemed to really get on and have seen each other whenever they can, usually in Los Angeles."
The two stars were recently seen together at the Chateau Marmont in Los Angeles.
The blonde beauty has been single since her split from actor Jude Law. Miller and Law shared a tumultuous, on-again, off-again relationship that ended for good last year.