The 29-year-old Paralympian was convicted of manslaughter – or culpable homicide – for shooting Steenkamp through a toilet door in his home early on Valentine’s Day 2013.
Prosecutors said Pistorius shot his girlfriend during an argument, while the defense said the athlete killed her by mistake, thinking she was an intruder.
The gold medal winner was sentenced to five years in jail last year, but only served a year behind bars at Pretoria Central Prison before being put under house arrest.
It is understood that he will spend remain under lock and key at home over Christmas, before being re-sentenced in the new year.
Judge Lorimer Leach’s decision depended on the legal concept of ‘dolus eventualis’ – whether someone foresaw that their actions could kill but pressed on anyway.
He told the court that “the principles of dolus eventualis were incorrectly applied” in his trial before asking “what should this court do in light of this finding?”
“He must have foreseen that whoever was behind the toilet door might die but reconciled himself to that event occurring,” he said.
And he branded the killing “a human tragedy of Shakespearean proportions” as he handed down his verdict.
The court, which has the power to overturn today’s ruling, could intervene if the killer appeals that his human rights have been infringed, for example.
Reeva’s mother June was heard to breathe a sigh of relief as the judge gave his verdict and was comforted by friends in court.
June, who left clutching a single red rose, has spoken of the injustice of the initial trial – but said she was unsure a longer sentence would make her feel better.