The actress, whose husband Peter Facinelli appears in the vampire franchise, told the New York Daily News that the British heartthrob was romancing an unidentified co-star.
"I can't say [who it is] because that would be breaking my promise to [my husband Peter], but he is dating one of them," she is quoted as saying.
Pattinson has earlier been linked to both Kristen Stewart (Bella Swan) and Nikki Reed (Rosalie Hale), although it is thought that Reed has begun dating Paris Hilton's ex-boyfriend Paris Latsis.
It was recently alleged that Pattinson and Stewart had been spotted attending a concert together in Hollywood, while Twilight director Catherine Hardwicke has also hinted at them being a real-life couple.
Both stars have previously denied the rumours.