The decision by the supreme Court of Cassation is the final ruling in the case, ending the long legal battle waged by Ms Knox and her ex-boyfriend.
Ms Kercher, 21, was sexually assaulted and stabbed to death in her bedroom in 2007 while studying in Perugia, Italy.
Arline Kercher, Meredith’s mother, from Coulsdon, Surrey, said she had heard little more about the decision other than the verdict.
She said: “(I am) a bit surprised, and very shocked, but that is about it at the moment.
“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.”
Ms Knox, who was Ms Kercher’s flatmate and a student from Seattle in the US, and Mr Sollecito spent four years in jail for the murder but were acquitted on appeal in 2011.
Ms Knox returned to the US before an appeal court threw out the acquittal and reinstated her and Mr Sollecito’s guilty verdicts last year.
But Italy’s highest court today overturned last year’s convictions and declined to order another trial,
Ms Knox said she was “tremendously relieved and grateful” at the decision, saying in a statement that knowing she was innocent gave her “strength in the darkest times of this ordeal”.
She added that she had relied on her family and friends and thanked her supporters, while her family expressed “profound gratitude” to those who had helped her.
Following the court’s decision, Ms Knox’s lawyer Carlo Dalla Vedova said: “Finished! It couldn’t be better than this.”
Ms Knox, who is now 27, awaited for the verdict in her home town of Seattle. Her Italian former boyfriend Mr Sollecito, 30, had his travel documents seized while the court proceedings were continuing.
The judges will release the reasons for their decision within 90 days after concluding that a conviction could not be supported by the evidence.
The Kercher family’s lawyer, Francesco Maresca, said earlier this week: “The interest of the family is to arrive to the end of this trial. They want to be able to remember Meredith outside of the courtroom.”
Prosecutors claimed that Ms Kercher was the victim of a drug-fuelled sex game gone wrong.
But Ms Knox and Mr Sollecito consistently protested their innocence and claimed they were not in the apartment the night she died.
Rudy Guede, a drug dealer, is serving a 16-year sentence over the death.
Ms Knox said last year she would become a “fugitive” if convicted and would have to be taken back “kicking and screaming” to Italy.
Last month she announced her engagement to 27-year-old musician and school friend Colin Sutherland, who wrote to her while she was in jail.
HOW THE TRAGEDY UNFOLDED
Amanda Knox and Raffaele Sollecito have had had their convictions overturned by Italy’s highest court, bringing to an end their legal battle following the murder of British student Meredith Kercher.
Here is a timeline of the case.
• November 2: Ms Kercher, a 21-year-old exchange student from Coulsdon, Surrey, is discovered with her throat cut in her bedroom at her house in the Italian town of Perugia. Her body is partially clothed and under a duvet.
• November 4: A post-mortem examination reveals evidence of sexual activity at some point before Ms Kercher died.
• November 6: Police arrest Ms Kercher’s American housemate Knox, then 20, Mr Sollecito, then 23, and Congolese Diya ‘Patrick’ Lumumba, who runs a local bar. Police claim Ms Kercher was murdered because she refused to take part in violent sex. Knox is said to have broken down and confessed and implicated Lumumba. The three are held on suspicion of conspiracy to commit manslaughter and sexual violence.
• November 11: Ms Kercher’s body is flown home. Knox’s mother, Edda Mellas, says Knox did not hear Ms Kercher’s screams the night she died and was with Sollecito at his house.
•: November 19: A fourth suspect is named as Rudy Hermann Guede, 20, from the Ivory Coast. He is thought to have left Perugia for Milan after Ms Kercher died.
• November 20: Guede is arrested in the German city of Mainz. Lumumba is released without charge.
• November 22: Guede admits being in Ms Kercher’s house on the night of the murder but says an Italian man he did not know committed the crime.
• September 9: Guede’s lawyers say he will ask to be prosecuted separately from Knox and Sollecito in a fast-track trial after talk of a possible pact between the former lovers to frame him.
• September 16: All three suspects appear before a judge in the first of a series of pre-trial hearings in Perugia. Judge Paolo Micheli grants Guede’s request for a fast-track trial.
• September 26: Knox and Sollecito come face to face in a closed courtroom for the first time since being detained after the murder.
• October 28: After 11 hours of deliberation, Judge Micheli sentences Guede to 30 years for the murder of Ms Kercher. He also orders Knox and Sollecito to stand trial for murder and sexual violence. Judge Micheli later rules that the pair remain in prison while they await trial.
• January 16: The trial of Knox and Sollecito begins.
• February 6: Sollecito tells the court he is not violent and has nothing to do with the case.
• June 6: Ms Kercher’s parents, John and Arline, give evidence. Mrs Kercher says she will never get over her daughter’s murder.
• June 12: Knox gives evidence in fluent Italian. She says she accused Lumumba “in confusion and under pressure” and that a police officer hit her during interrogation.
• November 21: Prosecutors ask for life sentences for Knox and Sollecito.
• December 4: Knox and Sollecito found guilty of murder. Knox is sentenced to 26 years and Sollecito to 25. Knox’s family say they will appeal.
• November 24: Knox and Sollecito return to court in Perugia for their appeal.
• December 11: Knox breaks down in tears as she makes an emotional courtroom appeal, saying she was the innocent victim of an “enormous mistake”.
• December 16: Italy’s highest criminal court upholds Guede’s conviction and prison sentence, which was slashed to 16 years in his first appeal.
• June 27: Guede gives evidence for the prosecution in the appeal and confirms the contents of a letter he wrote to his lawyers in 2010, which included a direct accusation against Knox and Sollecito.
• July 25: Experts tell the appeal court that forensic scientists who helped convict Knox made a series of errors. Evidence was tainted by the use of a dirty glove and failure to wear protective caps, they claim.
• September 7: Appeal court rejects prosecution request for new DNA tests.
• October 3: Knox is freed from prison after being acquitted of killing Ms Kercher. Sollecito is also cleared.
• October 4: Ms Kercher’s brother Lyle says her family accept the court’s decision but says questions remain unanswered about what really happened.
• February 16: Publisher HarperCollins announces it has signed a deal for a Knox memoir which was reportedly worth £2.5 million. The book, Waiting To Be Heard, is released in April 2013.
• April 29: Ms Kercher’s father John appeals to Guede to finally ‘’come clean’’ and reveal what really happened the night she was stabbed to death.
• March 26: Italy’s highest criminal court overturns the acquittals of Knox and Sollecito.
• September 30. The third trial of Knox and Sollecito begins in Florence.
• December 17: Knox declares her innocence in an email submitted to the appeal court in Florence by her lawyers before their closing arguments in which she says: ‘’I didn’t kill Meredith.’’
• January 30: The pair are found guilty of the murder of Ms Kercher after judges in Florence overruled their previous acquittals. Knox is sentenced to 28-and-a-half years and Sollecito to 25 years.
• March 25: Italy’s high court hears Knox and Sollecito’s appeal of the Florence conviction.
• March 27: After lengthy legal arguments Italy’s supreme Court of Cassation overturns the conviction and declines to order another trial. This is the final ruling in the case, ending the legal battle waged by Knox and Sollecito.