Pistorius killed Steenkamp by firing four bullets from a handgun through a closed toilet door in his luxury home in Pretoria on Valentine’s Day 2013. He claimed he mistook her for a burglar.
A spokesman for Ms Steenkamp’s family said the ruling “verified there was justice”.
In a sentence that was much lower than many had expected and was widely criticized, Pistorius had been sentenced to five years for culpable homicide in 2014 by high court judge Thokozile Masipa.
He served only 10 months of the five-year sentence in prison before being released and put under house arrest.
In 2016, following an appeal by the state against the culpable homicide conviction, the sentence was later replaced with murder by the supreme court of appeal. Pistorius was handed a six-year jail term in July of 2016.
The lower court had justified the six-year sentence by citing mitigating circumstances such as rehabilitation and remorse.
The Supreme Court of Appeal in Bloemfontein has now given him the minimum 15 years prescribed for murder in South Africa, less time already served.
Pistorius was not in court to hear the decision on Friday when the court handed it down.