Knox's former boyfriend Raffaele Sollecito has also been ordered to face a retrial over the 2007 killing and sexual assault of Meredith Kercher, Reuters wrote.
The ruling by the Court of Cassation, Italy's highest court, means the case against Knox, an American student, and Sollecito will be sent back to be reheard at a court in Florence, the Associated Press reported.
Knox and Sollecito were initially found guilty of killing Kercher, 21, in a drug-fueled sex game in the Italian university town of Perugia.
Kercher was found with her throat slashed and more than 40 wounds to her body.
Knox and Sollecito were sentenced to 26 and 25 years in prison respectively.
Their convictions were overturned by an appeals court in 2011 after forensic investigators challenged the police's scientific evidence, and they were released after spending four years in jail.
Meanwhile, an Ivorian living in Perugia, Rudy Guede, was tried separately and convicted, receiving a 16-year sentence.
The Court of Cassation upheld his conviction.
Prosecutors and lawyers for the Kercher family appealed against the acquittals, calling the verdicts "contradictory and illogical," Reuters wrote.
Knox returned to the US after her acquittal and it was not clear whether she would return to Italy for any re-trial.
Italian law does not require her to do so, the AP wrote. However, a New York Times report seemed to suggest that she could be extradited.
Knox, who is from Seattle, is now a student at the University of Washington studying creative writing, the AP cited her family spokesman, Dave Marriott, as saying.
She has written a book, "Waiting to be Heard," due out April 30, the same day she is set to be interviewed by Diane Sawyer of ABC’s "World News" in a world exclusive.