The South Africa Supreme Court of Appeal increased the sentence for Oscar Pistorious after the state argued that his original sentence of six years was "shockingly lenient".
The court handed down the minimum 15-year sentence prescribed for murder in South Africa, subtracting the year Pistorius has already served.
Pistorius was jailed in 2016 after being found guilty on appeal of murdering Ms Steenkamp on Valentine's Day 2013.
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.
"They just feel that their trust in the justice system has been confirmed this morning."