Italy's top appeals court has overturned the convictions of Amanda Knox and Raffaele Sollecito for the murder of UK student Meredith Kercher.
The decision is the final ruling in the long-running case.
The pair were found guilty in 2009, then freed in 2011 after the convictions were overturned. Their convictions were reinstated by another court last year.
Ms Kercher was found dead in 2007 in a flat she shared with Ms Knox.
'Full of joy'
The couple had always maintained their innocence and the decision by the Court of Cassation puts an end to their long legal battle.
The reasoning behind the decision will be made public in 90 days.
Ms Knox said she was "full of joy" after hearing the verdict.
"I'm still absorbing the present moment," she said, speaking outside her mother's house in Seattle, expressing thanks "for the justice I've received and for the support I've had from everyone".
"Meredith was my friend," she added, "she deserved so much in this life."
It has been a seven-year trial, and late on Friday night, after nine hours of discussion, five appeal court judges in Rome's Supreme Court stepped out to deliver a definitive acquittal of Amanda Knox and Raffaele Sollecito, in the murder of Meredith Kercher.
No reason was given. A brief statement was read by the presiding judge - justification will be delivered, in writing, within 90 days, in accordance with Italian law.
Neither defendant was in court to hear the acquittal in person, but there were cheers from both defence teams, who spoke of their surprise - an acquittal being a rarity in Italian Supreme Court hearings.
Lawyer Carlo Della Vedova broke the news to Ms Knox by phone, who cried with happiness, saying she now wants to "recover from this nightmare".
The Kercher family have not been in court for this hearing, but prosecution lawyer Francesco Maresca spoke of their concern that they won't ever learn the truth about what happened to Meredith eight years ago.
Prof Greg Hampikian, a friend of the family who also worked on the DNA evidence for Ms Knox's defence, told the BBC Radio 5 live that "everybody is very happy to see this finished, so they can get on with their lives".
"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 Rafaele and his folks," he said.
" I hope the Kerchers can find peace as well. It's just been an up and down thing for everyone for so long."
The lawyer for Ms Kercher's family, Francesco Maresca, has expressed his disappointment with the verdict.
"I think that it's a defeat for the Italian justice system," he told the Associated Press news agency.
Ms Kercher's mother has been quoted by the Press Association as saying that she is "surprised and very shocked" by the acquittals.
Twenty-one-year-old Ms Kercher was stabbed to death in the flat she shared with Ms Knox in Perugia, central Italy.
Her body was found under a duvet in her bedroom, which had been locked from the inside.
Days after the killing, Ms Knox and her Italian boyfriend at the time, Raffaelle Sollecito, were arrested.
They were convicted of murder in 2009 by a court in Perugia.
Whoops of joy erupted from the family home in the sleepy Seattle suburb of Arbor Heights as the verdict was announced.
All morning family and friends had been coming and going - members of the 'Friends of Amanda' campaign that has lobbied so tirelessly for her release.
They and many others in America's Pacific North-West have long been convinced Amanda Knox is the victim of a major miscarriage of justice.
One supporter, former Judge Michael Heavey, likened it, in an interview with the BBC, to a "modern day witch hunt".
Amanda Knox is now free to travel outside the US, and, at the age of 27, to plan for her future - a future free of the risk of being extradited to Italy to serve out the remainder of a 28-and-a-half year sentence.
But in 2011, a jury cleared both defendants of the charge, after doubts were raised over the handling of DNA evidence.
A retrial was ordered after prosecutors argued that important evidence had been disregarded. In 2014, the guilty verdict was reinstated.
Another man, Rudy Hermann Guede, born in Ivory Coast, was also convicted in a separate trial and is serving a 16-year sentence for his role in the murder.