To date, he has served just over a year of that six-year sentence. At the Supreme Court of Appeal, Prosecutors argued that the six-year sentence was "shockingly lenient".
Pistorius, a gold medal-winning athlete, is known as the "Blade Runner" for his carbon-fibre prosthetics.
After a high-profile trial, he was in 2014 convicted of culpable homicide and sentenced to five years in jail, of which he served 10 months before being put under house arrest.
"They feel there has been justice for Reeva". He was imprisoned a year ago.
Pistorius' legal counsel said the claims that his client is not remorseful are false.
The accused's brother Carl Pistorius tweeted he was "shattered, heartbroken, gutted" by the ruling.
As reported by News 24, Justice Legoabe Willie Seriti told the court, "The sentence imposed by the...[high court] with respect to murder is set aside and substituted with the following - the respondent's imprisonment for 13 years and five months".
The new sentence was backdated to start on the day he began his murder sentence, on July 6 a year ago. Fifteen years is the country's usual minimum sentence for murder. "Leave to appeal should not be granted".
He was sentenced to six years.
Pistorius, who turned 31 on Wednesday, is being held at the Atteridgeville Correctional Centre on the outskirts of the South African capital, Pretoria, and did not attend any of the appeal hearings.
Pistorius can appeal the new sentence, according to Kelly Phelps, an adviser to his defense team, but no decision has been made. Pistorius might now be moved back to a higher security facility.
Pistorius was jailed in July 2016 after being found guilty on appeal of murdering Ms Steenkamp on Valentine's Day 2013.