In July 2016, South Africa's National Prosecuting Agency deemed the sentence "shockingly too lenient", announcing plans to appeal for a longer sentence.
Justice Willie Seriti of South Africa's Supreme Court of Appeal announced Friday that the five-judge panel had upheld prosecutors' appeals, according to The Associated Press, and increased the sentence to 15 years, minus time served.
Judge Thokozile Masipa ruled the state failed to prove beyond reasonable doubt that Pistorius was guilty of premeditated murder, but may still be guilty of culpable homicide - equivalent to the United Kingdom law of manslaughter.
"It is shockingly lenient and has accordingly resulted in an injustice", she said, referring to the sentence.
"They feel there has been justice for Reeva". Claiming he mistook his girlfriend for an intruder, he was initially convicted of manslaughter.
Image Reeva Steenkamp and Oscar Pistorius in 2013
The BBC reports that a lower court had "justified the six-year sentence by citing mitigating circumstances such as rehabilitation and remorse".
The ruling added that Pistorius "displays a lack of remorse and does not appreciate the gravity of his actions". The court agreed and the sentence is now 13 years.
Attorney Marius du Toit, a legal expert in South Africa, told News 24 he thought the 13 years was "a heavy sentence".
Supreme Court judges are generally reluctant to change sentences handed down by trial courts, and it's rare for them to change one so dramatically.
But even though State prosecutors managed to get it upgraded to murder, only one extra year was added to the sentence. Pistorius won't be eligible for parole for at least six years. 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.