The 31-year-old's sentence has been increased to 13 years and five months by South Africa's Supreme Court of Appeal (SCA) in Bloemfontein.
The court upheld an appeal by prosecutors, who challenged the original six-year sentence for being too lenient over the shooting.
The Paralympic athlete, who won gold in the 100m relay at London 2012, has so far served a year of the original sentence.
Pistorius shot Steenkamp in the early hours of Valentine's Day in 2013. He was arrested that same day and later convicted of manslaughter, which was later replaced with a murder conviction by the Supreme Court.
A spokesperson for Steenkamp's family said the ruling had "verified there is justice".
Prosecutors had told the court Pistorius' disability had been "overemphasised" by a judge and that the original 2016 sentence was far too lenient on the double-amputee.
“Murder is murder,” chief prosecutor Andrea Johnson told a five-judge panel. She asked them to overturn Pistorius’s initial sentence and give him the prescribed minimum of 15 years. There is no death penalty in South Africa.
This is the second time prosecutors have gone to the Supreme Court to challenge a decision by Judge Thokozile Masipa, who presided over Pistorius’s trial.
In 2015, the state successfully appealed against Ms Masipa’s judgement that Pistorius was not guilty of murder. The court overturned her verdict of culpable homicide – or manslaughter – and convicted Pistorius of murder.
Pistorius has claimed he mistook Steenkamp for an intruder hiding in his bathroom during the morning of the shooting. Without his prosthetic legs on and standing on his stumps, he shot four times through the cubicle door.
Join our new commenting forum
Join thought-provoking conversations, follow other Independent readers and see their replies