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 was born in Minneapolis, Minnesota, in Hennepin County Medical Center, the daughter and only child of Russian immigrants Dr. Larissa Vassilieva, a biologist, and Dr. Vladimir Vassiliev, a physicist, both from Novosibirsk. She attended and graduated from Barbizon Modeling and Acting School in Arizona.
At the age of seven, Vassilieva was discovered at the International Modeling and Talent Association (IMTA) in New York, where she won the title of Child Actress and first runner-up Child Model Of The Year 2000.