The state has argued that the former Olympian’s original sentence of six years was "shockingly lenient".
In a case that attracted worldwide interest, Pistorious was imprisoned in July last year after being found to have shot Steenkamp, a model and law graduate, in the early hours of Valentine's Day 2013, saying he mistook her for a burglar when he fired four times through the door of his bedroom toilet.
He was originally convicted of manslaughter and sentenced to five years in jail.
The athlete was not in court for Friday's ruling where the court handed down the minimum 15-year sentence prescribed for murder in South Africa, and subtracted the years Pistorius had already served in jail.
The Steenkamp family welcomed the increased sentence, saying it showed that justice could prevail in South Africa.
"This is an emotional thing for them. They just feel that their trust in the justice system has been confirmed this morning," Tania Koen, a spokeswoman for the Steenkamp family, told Reuters.
In an interview with SABC television, Reeva’s mother June Steenkamp said: "We felt that we didn't have justice for Reeva by that too-lenient sentence, but now we have justice for her."
In December 2015, the Supreme Court of Appeal ruled Pistorious was guilty of murder when he opened fire with a high-calibre pistol he kept under his bed.
The athlete had the lower part of his legs amputated when he was a baby and his lawyers argued that his disability and mental stress should be considered as mitigating circumstances to reduce his sentence.