To date, he has served just over a year of that six-year sentence.
Pistorius claimed he shot dead Steenkamp on Valentine's Day after mistaking her for a burglar.
After Judge Masipa initially refused leave to appeal against the sentence, the State approached the SCA, arguing that six years imprisonment for murder was shockingly inappropriate and would set a bad precedent.
In July 2016, South Africa's National Prosecuting Agency deemed the sentence "shockingly too lenient", announcing plans to appeal for a longer sentence.
According to The Guardian, Pistorius, who wasn't present in court, had his sentence increased by the Supreme Court of Appeal in Bloemfontein on Friday, where a 15-year sentence was handed down by the court - the minimum sentence for murder in South Africa - minus the term Pistorius had already served in jail.
Pistorius, a gold medal-winning athlete, is known as the "Blade Runner" for his carbon-fibre prosthetics.
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However, the southpaw has not been ruled off the Test yet and remains available for selection. Marsh is now a part of Australia's squad for the first two Ashes Tests. "He'll be okay.
The accused's brother Carl Pistorius tweeted he was "shattered, heartbroken, gutted" by the ruling.
Pistorius, the first amputee runner to compete in the Summer Olympics at the London 2012 Games, was banned by the International Paralympic Committee for five years in 2014 when his murder trial began.
Attorney Marius du Toit, a legal expert in South Africa, told News 24 he thought the 13 years was "a heavy sentence".
The new sentence was backdated to start on the day he began his murder sentence, on July 6 past year.
Pistorius must serve at least half of the 13 years and five months - almost seven years - before he can be considered for parole.
But even though State prosecutors managed to get it upgraded to murder, only one extra year was added to the sentence. Pistorius was moved from the high security Kgosi Mampuru II Prison in central Pretoria to Atteridgeville, which houses prisoners sentenced to six years or less.