Oscar Pistorius is now a convicted murderer after a South African court overturned a previous manslaughter charge.
The athlete killed his girlfriend Reeva Steenkamp on Valentine's Day in 2013 after shooting her through a door at his home in Pretoria.
He was jailed for five years last year after a judge found him guilty of "culpable homicide".
But prosecutors appealed against the conviction, arguing that the athlete should have been found guilty of murder.
Their appeal was successful and Pistorius will now have to return to court to be re-sentenced for murder.
- What did the judge say?
Justice Lorimer Leach said the original verdict was "flawed".
The prosecution appeal focused on the rule of dolus eventualis - whether Pistorius knew that a death would be a likely result of his actions.
He said Pistorius must have foreseen that whoever was behind the door might die when he armed himself with a high-calibre weapon.
The defence argued that he believed that there was an intruder in the house but the judge said that the identity of the person behind the door was irrelevant.
- What does this mean for Pistorius?
The athlete is currently under house arrest at his uncle's luxurious home in Pretoria after serving less than a year for killing his girlfriend on Valentine's Day 2013.
Judge Thokozile Masipa, who originally found Pistorius guilty of South Africa's equivalent of manslaughter, will re-sentence the Paralympic champion at a later date.
A murder conviction carries a minimum of 15 years in jail in South Africa, although Pistorius' lawyers are likely to argue that his physical disability and mental stress should be considered as mitigating circumstances.
It is not yet clear whether he will return to prison immediately, or if he will be allowed to remain in correctional supervision until a new sentence is handed down.
- How did Reeva Steenkamp's family react?
Reeva Steenkamp's mother, June, was in court for the hearing. She closed her eyes as the decision was read out in the Bloemfontein court and hugged a friend.
She later spoke to NBC News, saying she finally feels that justice has been done.