The judge described the Paralympian's testimony over the 2013 shooting of Reeva Steenkamp as "untruthful" and delivered a damning indictment of the original verdict.
The double-amputee athlete known as the "Blade Runner" was released on parole in October after serving one year of his five-year prison sentence for the lesser charge of culpable homicide.
He killed Steenkamp, a model and law graduate, in the early hours of Valentine's Day two years ago, saying he mistook her for an intruder when he shot four times through the locked door of his bedroom toilet.
"The matter is referred back to the trial court to consider an appropriate sentence."
Also Read
The Paralympic gold medallist, who was not present in court, now faces a minimum 15-year sentence for murder -- although he could be released earlier on parole.
"It is inconceivable that a rational person thought he was entitled to fire at this person with a heavy-duty firearm," said Leach, who described Pistorius's testimony as "vacillating and untruthful".
"He ought to have been convicted not of culpable homicide... But of culpable murder," Leach added in a damning rejection of Pistorius's evidence and the original trial judge's ruling.