Sofia Vladimirovna Vassilieva (Russian: София Владимировна Васильева; born October 22, 1992) is an American actress. Her most notable roles include the children's book Eloise in Eloise at the Plaza and Eloise at Christmastime, Ariel DuBois in the Emmy-winning TV series Medium, and teenage cancer patient Kate Fitzgerald in the 2009 film adaptation of My Sister's Keeper by Jodi Picoult.Vassilieva played the lead role of terminally ill teenager Kate Fitzgerald in the 2009 adaptation of Jodi Picoult's novel My Sister's Keeper, directed by Nick Cassavetes. She starred alongside Cameron Diaz, Jason Patric, and Abigail Breslin. Vassilieva shaved her head and eyebrows for her role as a teenage cancer patient. In the same year, she became an honorary ambassador for Stand Up to Cancer. For her performance as Kate, Vassilieva won a Young Artist Award for Best Supporting Actress in a Feature Film in 2010. It was her third nomination and second win.

In 2011, she guest-starred on Law & Order: SVU as Sarah Walsh, a piano BFA candidate who was raped in her New York City apartment. She reprised her role as Sarah in 2013.