Sofia Vladimirovna Vassilieva (born October 22, 1992) is an American actress. Notable roles include portraying the children's book character 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.

Early life
Vassilieva was born on October 22, 1992 in Minneapolis, Minnesota, in Hennepin County Medical Center, the only child of Russian immigrants. She attended and graduated from Barbizon Modeling and Acting School in Arizona.