In an announcement that took a matter of minutes, Supreme Court Justice Willie Seriti said the Supreme Court upheld an appeal by prosecutors against Pistorius' original six-year sentence for shooting Steenkamp multiple times in his home in 2013.
Prosecutors had called that six-year sentence "shockingly" lenient.
Pistorius should have been sentenced to the prescribed minimum of 15 years for murder in South Africa, Seriti said, as he delivered the verdict that was reached by a panel of five judges at the Supreme Court in the central city of Bloemfontein.
The new sentence of 13 years and five months took into account time Pistorius had already served in prison and at home under house arrest, Seriti said.
Pistorius, who turned 31 on Wednesday, has served over a year of his initial six-year sentence.
Pistorius killed Steenkamp in the pre-dawn hours of Valentine's Day 2013 after shooting four times through a closed toilet cubicle door in his home. Claiming he mistook his girlfriend for an intruder, he was initially convicted of manslaughter. That conviction was overturned and replaced with a murder conviction by the Supreme Court in 2015.
Friday's decision likely brings an end to a near five-year legal saga surrounding the double-amputee athlete, a multiple Paralympic champion and record-breaker who was once one of the most celebrated sportsmen in the world.
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.
Pistorius failed with an appeal to the Constitutional Court last year to challenge his murder conviction.
