The court found 22-year-old Amanda Knox guilty of murder and sexual assault in the 2007 slaying of Meredith Kercher during a drunken sex game.
The eight members of the jury also convicted Knox's Italian ex-boyfriend, Raffaele Sollecito, of the same charges, sentencing him to 25 years.
All three were studying in Perugia at the time.
Kercher's body was found on Nov 2, 2007, semi-naked with her throat slit in the bedroom of the house she shared with Knox.
Knox's lawyer and members of her family say they will file an appeal against the verdict.
They alleged that the three young people were high on drugs when they tried to persuade Kercher to take part in a sex game that turned violent.