The surprise decision definitively ends the 7½
-year legal battle waged by Knox, 27, and co-defendant Raffaele Sollecito, 31, to clear their names in the gruesome 2007 murder and sexual assault of British student Meredith Kercher.
Meredith Kercher's mother said she was "surprised and very shocked" after Italy's top court cleared Amanda Knox and Raffaele Sollecito of her daughter's murder.
Arline Kercher said the decision Friday by the Court of Cassation in Rome was "odd" given the pair had been convicted twice following the British student's killing in 2007.
Judges cleared Knox and Sollecito after 10 hours of deliberations, concluding an eight-year legal drama.
Arline Kercher said she had heard little more about the decision other than the verdict.
"(I am) a bit surprised, and very shocked, but that is about it at the moment," she said.
"They have been convicted twice so it's a bit odd that it should change now."
Asked whether she had any plans following the ruling, she said: "I really don't know at the moment, I haven't got any plans."
Kercher, who was 21 and from the south London suburb of Coulsdon, was sexually assaulted and stabbed to death in her bedroom while studying in Perugia.
Knox and Sollecito were convicted for a second time last year for taking part in the brutal slaying of the British student, with whom Knox, then 20, shared a house in the university town.
Italian Sollecito was the American's boyfriend. The pair served four years in prison. Ivory Coast-born drifter Rudy Guede was also jailed for the murder.
A family friend of 27-year-old Knox, who has spoken on the phone to her family at their Seattle home since the verdict, told BBC radio that "everybody is very happy to see this finished, so they can get on with their lives".
Greg Hampikian, a professor of biology and criminal justice at Boise State University in Idaho, worked on the DNA evidence for Knox's defence team.
"I imagine she is feeling a lot better -- she was under tremendous stress before this, and I think it was really starting to wear on Amanda," he said.
"She's trying to start her life as a young woman, so hopefully this will be a really wonderful change and a new day for them and for Raffaele and his folks.
"I hope the Kerchers can find peace as well. It's just been an up and down thing for everyone for so long."
He also criticised the Italian investigation, saying: "The evidence in this case clearly points to one perpetrator. The only DNA they found was the victim's and Rudy Guede's, and that should have been open and shut.
"The fact that they persisted in accusing two people against whom there was such slim evidence was really just a very bad way to continue with the case. It's fine to start with a hunch but you've got to drop it when you see the bad data."