Prosecutors said the couple had killed Meredith Kercher in November 2007. They were convicted two years later of murder, but those charges were overturned on appeal in 2011.
A judge said Thursday that Knox was sentenced to 28 1/2 years in prison. Sollecito's sentence was 25 years.
It is unlikely that Knox, who lives in Seattle, Washington, will return to Italy to serve additional prison time because U.S. law dictates that a person cannot be tried twice on the same charge, a legal expert told CNN. He believes that if Italy were to ask for extradition, U.S. officials would deny the request.
"She was once put in jeopardy and later acquitted," said Sean Casey, a former prosecutor who is now a partner at Kobre & Kim in New York. "Under the treaty, extradition should not be granted."
Presiding judge Alessandro Nencini has 90 days to write his arguments behind the jury's ruling. Once that is out, lawyers have 90 days to appeal.
Kercher, 21, of Great Britain, was found partially naked in a pool of blood in the house she shared with Knox in the picturesque town of Perugia, where both women were exchange students.
Knox has said she is afraid to return to Italy, where she spent four years behind bars.
"I will become ... a fugitive," she told Italian daily La Repubblica this month, when asked what she would do if she was found guilty in the second trial.
Italy's Supreme Court in March overturned the pair's acquittals, saying that the jury did not consider all the evidence and that discrepancies in testimony needed to be answered.
The case was sent to a retrial in Florence.
Dressed in a purple sweater with sunglasses nestled on his collar, Sollecito made it to the tribunal for the morning session and was expected to return for the verdict, but his attorney, Luca Maori, said Thursday evening that Sollecito would not be coming back to court.
The retrial began in September, refocusing international attention on the case that grabbed headlines in Italy, Britain and the United States -- but neither Knox nor Sollecito were present in court.
It has renewed questions about the effectiveness of Italy's justice system, given widespread doubts over the handling of the investigation and key pieces of evidence.
Both Knox and Sollecito have maintained their innocence.