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, also convicted of slander, was sentenced in absentia to 28 1/2 years in prison. Sollecito's sentence was 25 years.
Knox, who was at home in Seattle, Washington, said her conviction would bring no consolation to the Kercher family.
"I am frightened and saddened by this unjust verdict," she said in written remarks. "Having been found innocent before, I expected better from the Italian justice system. The evidence and accusatory theory do not justify a verdict of guilt beyond a reasonable doubt. ...There has always been a marked lack of evidence."
She called the legal proceedings a travesty.
"This has gotten out of hand. Most troubling is that it was entirely preventable," she said. "I beseech those with the knowledge and authority to address and remediate the problems that worked to pervert the course of justice and waste the valuable resources of the system."
Knox also said that Kercher's family had suffered greatly.
"Their grief over Meredith's terrible murder will follow them forever. They deserve respect and support."
What comes next
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.
Knox's attorney, Ted Simon, said there will certainly be an appeal and cautioned that extradition shouldn't yet be a part of the conversation surrounding the case.
"It's really not in play right now, because first of all, she has another appeal to the Supreme Court of Italy," he told CNN's Anderson Cooper 360. "In Italy, under their system, you're still actually presumed innocent until that third, final stage."
Simon said that if extradition does become an issue, Knox has "very substantial defenses" that can be used.
"But I think we shouldn't get ahead of ourselves," he told CNN. "The bottom line is, there is no evidence. There was no evidence, and there never will be any evidence, and that's why this is such a gross miscarriage of justice."
Legal analysts debated whether the U.S. would send Knox back to Italy if Rome requests extradition.
It is unlikely that Knox 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.
"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 (between the two nations), extradition should not be granted."
CNN legal analyst Mark O'Mara said that the United States has to respect the treaty.
"We have to follow the letter of the law," he said Thursday.
Knox says she won't go back
Kercher, 21, of Great Britain, was found partially nude 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.
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.
Knox, 26, and Sollecito, 29, were convicted in 2009 of killing Kercher, who was found with more than 40 stab wounds and a deep gash in her throat.
Prosecutors say she was held down and stabbed after she rejected attempts by Knox, Sollecito and another man, Ivory Coast-born Rudy Guede, to involve her in a sex game. Guede is the only person in jail for the murder, and many aspects of the crime still remain unexplained.
Speaking on Thursday, Knox's defense team asked for an acquittal.
Knox has always denied murdering Kercher and has maintained she is not guilty in a written statement to the Florence court.
"I must repeat to you. I'm innocent. I did not rape, I did not steal ... I did not kill Meredith," Knox said in a lengthy e-mail presented by her lawyer to the court in December.
Sollecito was in the Dominican Republic at the start of the retrial but returned to Italy.
In November, he took to the stand to make a spontaneous declaration, saying the charges against him were "absurd."
"For me, it's a nightmare that goes beyond imagination," he said of what he's been through.
'No one remembers Meredith'
The case has dragged on for more than six years, frustrating attempts by Kercher's family to discover the truth about her death. The three trials have done little to clear up mysteries surrounding the details of the murder.
"They are tired of this long trial and they want justice," Francesci Maresca, attorney for the Kercher family, said.
The Kercher family welcomed the retrial ruling, Maresca said in March, adding they believed the ruling that acquitted Knox and Sollecito was "superficial and unbalanced."
They believe more than one person was in the room when Kercher was killed, he said.
"No one remembers Meredith, while the two defendants write books, speak to the media and earn money," Maresca told the court in closing remarks last month.
CNN's Hada Messia reported from Florence, Italy. Steve Almasy reported and wrote from Atlanta. CNN's Livia Borghese. Elwyn Lopez and Alexa Miranda contributed to this report