Pistorius killed his girlfriend Reeva Steenkamp in February 2013 after shooting four times through a locked toilet door. He is currently under house arrest after spending one year of his original five-year sentence in jail.
Pistorius will have to return to court to be re-sentenced, for murder.
It was earlier incorrectly reported that the court had ruled the manslaughter verdict would remain.
South Africa's Supreme Court of Appeal ruled that the lower court did not correctly apply the rule of dolus eventualis - whether Pistorius knew that a death would be a likely result of his actions. The minimum sentence for murder in South Africa is 15 years, but judges can apply some discretion.
South African law does not make provision for someone to be placed under house arrest for more than five years, so Pistorius will be going back to prison, reports the BBC's Pumza Fihlani in Johannesburg.
He will be back behind bars, less than two months after he was placed under house arrest.
When will he be sentenced? We don't have a date yet, but it will be next year. The minimum sentence for murder is 15 years, but the judge does have the discretion to lower it.
Can he appeal? Yes, but only if his lawyers are convinced that the appeal judges violated his constitutional rights. So it's a high threshold, and hard to meet.
So is this the end of Pistorius' professional athletics career? Almost certainly. He's 29, and will be past his prime by the time he is freed. It is also unlikely that advertisers would want to sponsor him, as the Pistorius brand is now tainted. The making and unmaking of Oscar Pistorius
Pistorius' family gave a brief response, saying lawyers are studying the finding who will advise them on "options going forward". The panel of appeal judges described the case as "a human tragedy of Shakespearean proportions" in their written judgement. Reading the unanimous ruling reached by the five judges, Justice Eric Leach said that having armed himself with a high-calibre weapon, Pistorius must have foreseen that whoever was behind the door might die, especially given his firearms training.
"As a matter of common sense at the time the fatal shots were fired, the possibility of the death of the person behind the door was clearly an obvious result," the judge said.
"And in firing not one but four shots, such a result became even more likely."
Pistorius always maintained that he believed
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