Knox, 22, and 25-year-old Raffaele Sollecito were in the dock to hear the jurors deliver their verdict. Knox was sentenced to 26 years in prison and Sollecito 25. The pair are expected to appeal against the decision.
Meredith Kercher, 21, from Coulsden, Surrey, had been studying at Perugia's University for Foreigners. She was found dead on 2 November 2007 in the bedroom of the house she shared with fellow student Knox.
Prosecutors alleged that Sollecito held Kercher down while Rudy Guede, a 22-year-old Ivory Coast-born drifter, tried to sexually assault her and Knox stabbed her. In October 2008 Guede was given a 30-year sentence for the murder after opting for a fast-track trial. He is appealing against his conviction.
Knox, from Seattle, denied murdering Kercher, and on Thursday appealed to the jury not to convict her, saying she "was confident my conscience is clean."
Using the fluent Italian she has learned during two years in jail, Knox said: "I could be pulling out my hair, taking apart my cell but I don't do these things. I just take a breath and try and be positive in moments like this." Turning to the judges and jury, she said: "Now it's your turn and I thank you."
Prosecutors claimed that Knox's DNA was found on the handle of the likely murder weapon – a kitchen knife found in Sollecito's house – and that traces of Kercher's DNA were on the blade. Knox said she had spent the night before Kercher's body was found at Sollecito's flat, and was shocked when she realised police suspected her.
Knox and Sollecito were remanded in custody shortly after the killing, when they gave conflicting statements over their whereabouts on the night of the murder. Sollecito said he was at his flat in Perugia using his computer, and he did not remember whether Knox spent the whole night with him or just part of it.
During the trial, Knox claimed that she had been mistreated by Italian police who questioned her. She said a policewoman hit her on the head twice during interrogation. Knox claimed police pressure initially led her to name Diya "Patrick" Lumumba as the culprit.
Lumumba, a Congolese man who owned a pub in Perugia, was held briefly in the case but later cleared. He is seeking damages from Knox for defamation.
"It was always a crescendo," Knox told the court. "When I said I was with Raffaele all the time they told me I was a liar. I was scared, I thought: maybe they are right."