The Supreme Court of Appeals has found the athlete guilty of murder, overturning his culpable homicide conviction for shooting dead his girlfriend.
Oscar Pistorius fatally shot his girlfriend Reeva Steenkamp through a toilet door in 2013.
His defence in his trial was that he thought she was an intruder, and last year he was given a five-year jail sentence for "culpable homicide."
The original ruling by Judge Thokozile Masipa was partly based on the finding that Pistorius did not know whether the shots he fired would kill someone.
That, according to Supreme Court Justice Eric Leach, was incorrect.
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He says regardless of who was behind the door, Pistorius could foresee the consequences of firing repeatedly through the door.
"Although frightened, the accused armed himself to shoot if there was someone in the bathroom. And when there was, he did. In doing so he must have foreseen that the person he was firing at behind the door might be fatally injured. Yet he fired without having a rational or genuine fear that his life was in danger. The defence of self defence cannot be sustained."
Mr Leach said Pistorius' decision to shoot at the toilet door without knowing if he was in danger was irrational-- and that the identity of the person behind the door was irrelevant.
"As a result of the errors of law referred to and on a proper appraisal of the facts, he ought to have been convicted not of culpable homicide on that count, but of murder. In the interest of justice, the conviction and sentence imposed must be set aside and the conviction must be substituted with a conviction of the correct offence."
Despite his decision, Justice Leach concluded the 53 minute-long ruling by praising the conduct of the original trial judge.
Pistorius, who was not present in court for the new ruling, has been under house arrest after spending one year of his sentence in jail.
But now he will have to return to court to be re-sentenced, and could spend as many as 15 years behind bars.
Ms Steenkamp's mother, June, says the athlete must take responsibility for what he's done.
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"I really don't care how long it is, as long as he pays for what he's done and he takes responsibility. We put our faith in the justice system but now we can be assured that we're going to get justice. Because we have, even after three years, of trying for justice, we've got justice. We've got respect for Reeva, we've got respect for women, and I think there is a lack of respect for women in this country."
On the streets of Bloemfontein, where the ruling was handed down, most South Africans are welcoming the new ruling.
"Today's judgment, I think it goes back, it goes far, in making sure that people cannot just kill other people and claim self-defence and give all false reasons. Justice has been served, my brother, that's the long and the short of what I can tell you."
