The verdict overturns they twosome's 2011 acquital, which freed them after four years in jail. Now both Knox and Sollecito -- who pleaded not guilty to the murder -- face long jail sentences.
Thursday's appeal took place in Florence and was ordered by Italy's highest court, the supreme court in Rome. After four months of deliberations, the jury found that the the appeal that freed the defendants in 2011 was flawed.
The 2011 trial shed light on many DNA incongruences that were overlooked during the pair's first trial and conviction in 2009.
Kercher -- Knox's roommate in Perugia -- was found partially naked in a pool of blood in the house she shared with the Seattle native. Her siblings Lyle and Stephanie were present when the verdict was read out Thursday.
Both Knox and Sollecito have maintained their innocence from the time they were arrested soon after Kercher's body was found stabbed to death.
Knox, who did not return to Italy for the hearing, has stated that if convicted she would become a "fugitive" but would "continue to fight" for her innocence. Sollecito was in attendance during Thursday's trial.
Rudy Guede, who was born in the Ivory Coast but raised in Perugia, was also convicted of Kercher's murder in 2008. Prosecutors initially believed he murdered Kercher in a sex game with Sollecito and Knox. They eventually changed their argument and claimed Knox murdered her roommate over a heated arguments over cleanliness in the Perugia apartment they shared.