Oscar Pistorius is facing a minimum of 15 years in jail after his original conviction of manslaughter for killing Reeva Steenkamp was changed to murder.
Pistorius will have to return to court for re-sentencing after his earlier conviction for the lesser crime of culpable homicide - equivalent to manslaughter - was thrown out on appeal.
The six-time Paralympic gold medallist had been under house arrest at his uncle's home in Pretoria after serving less than a year in prison for killing his girlfriend on Valentine's Day in 2013.
State prosecutors had appealed to the Supreme Court in Bloemfontein against the ruling of culpable homicide in the original trial.
The state argued the original trial judge misinterpreted the law of 'dolus eventualis' - a Latin legal phrase meaning the act was carried out by the accused knowing it would lead to death and the accused went through with it anyway.
Reeva Steenkamp was killed on Valentine's Day 2013
In this case the state argued that Pistorius knew that the act of firing his gun through the door of his bathroom would lead to death.
On Thursday morning, with Ms Steenkamp's mother June watching on, Judge Eric Leach announced that the state prosecutors' appeal was upheld, that the original ruling was "flawed" - and that Pistorius was guilty of murder.
Judge Leach was part of a panel of five judges who ruled on the appeal.
Pistorius has won six gold medals at the Paralympic Games
"Guilty of murder, with the accused having criminal intent," judge Leach told the court. "The matter is referred back to the trial court to consider an appropriate sentence."
Pistorius said during his trial last year that he mistook Steenkamp for an intruder when he opened fire at the locked door of his bedroom toilet.
However, that testimony was rejected, as was the original trial judge's ruling.
Pistorius was originally convicted of culpable homicide
"He (Pistorius) did not know whether that person (behind the door) constituted any threat," said Leach.
"It is inconceivable that a rational person thought he was entitled to fire at this person with a heavy-duty firearm. He must have foreseen that the person behind the door might be injured.
"He ought to have been convicted not of culpable homicide on that count but of culpable murder."
Pistorius competed in the able-bodied Olympics in 2012
Pistorius will stay under house arrest until his sentencing, which is likely to take place in January or February.
The victim's father, Barry Steenkamp, said: "I'm satisfied with everything now. I would hope to God that all of this could have been prevented, but seeing that it has been done, let us now all get on with our lives."
Pistorius' legal team said: "We have taken note of the judgement that has just been handed down by the Supreme Court of Appeal. The legal team will study the finding and we will be guided by them in terms of options going forward."
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