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.
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.
The BBC reports that a lower court had "justified the six-year sentence by citing mitigating circumstances such as rehabilitation and remorse". Fifteen years is the country's usual minimum sentence for murder.
The athlete was originally convicted of manslaughter and sentenced to five years in jail.
The runner claimed he mistook 29-year-old Ms Steenkamp for an intruder when he fired four bullets through the bathroom door in the early hours of 14 February 2013.
Tania Koen, a spokeswoman for the Steenkamp family, said: "This is an emotional thing for them".
The ruling added that Pistorius "displays a lack of remorse and does not appreciate the gravity of his actions".
Pistorius' lawyers have just one avenue open to them if they want to challenge the new sentence handed down by the Supreme Court, and that is to appeal to the Constitutional Court, the highest court in South Africa. He served 10 months of a five-year sentence before being placed under house arrest.
"Leave to appeal should really not be granted", he said.
Oscar Pistorius sits in the dock during his sentencing hearing at North Gauteng High Court on June 13, 2016 in Pretoria.
At the same time, Steenkamp's parents have sent their love to Jayde Panayiatou's parents after her husband, Chris, was sentenced to life in prison for her murder.