An appeals court in Florence had last year handed Knox a three-year sentence for wrongly accusing Congolese bar owner Patrick Lumumba of killing Meredith Kercher in the city of Perugia.
Knox, 37, served four years in jail for the killing of Kercher before the conviction was annulled in 2015.
She was aiming to clear her name in Rome's Court of Cassation in the last legal case against her over the affair.
The sentence has no practical effect as it was covered by the time Knox spent in prison.
Lumumba welcomed the verdict.
"I am very satisfied. Amanda did wrong, this sentence must accompany her for the rest of her life. I had a good feeling about this since the afternoon. I hail Italian justice with great honour," Lumumba said.
Knox, who lives in the United States, did not attend the court.
Her lawyers expressed their surprise at the outcome.
"We cannot believe it. A totally unjust decision for Amanda and unexpected in our eyes. We are incredulous, we take note and will read the motivations," Knox's lawyer Luca Lupària Donati said.
Knox had protested her innocence in a series of posts on X before the hearing on Thursday.
"It doesn't get easier, no matter how many times I've been through this," she wrote.
"I am not a liar. I am not a slanderer."
The stabbing of 21-year-old Kercher and multiple trials provided fodder for tabloids on both sides of the Atlantic and inspired books and films.
Rudy Guede, originally from the Ivory Coast, was sentenced to 16 years in jail for the killing of Kercher, in a ruling that said he acted with unnamed other culprits.
He was granted early release in 2021.
