South Africa’s Supreme Court of Appeal on Friday more than doubled the sentence imposed by a lower court on Oscar Pistorius, a double-amputee Olympic sprinter convicted of killing his girlfriend, Reeva Steenkamp, in his home in February 2013.

The case, evoking questions about racial fears and violence against women, transfixed South Africa’s tangled, post-apartheid society — and a broader international audience. Ms. Steenkamp, 29, was a model and law graduate, and Mr. Pistorius, now 31, had been an emblem of triumph over adversity after the amputation in his infancy of both legs below the knee. They had been dating for several months and became known as a celebrity couple.

After his initial trial in 2014, Mr. Pistorius, sometimes nicknamed Blade Runner for the curved prosthetics on which he runs, was sentenced for manslaughter, but that was later changed on appeal to murder. He was jailed for six years in July 2016.

Prosecutors said the six-year term was “shockingly lenient,” however, and they appealed. The Supreme Court of Appeal increased the sentence on Friday to 15 years but deducted the time he has already served either in prison or under house arrest, meaning that he faces an outstanding sentence of 13 years and five months. South African law sets 15 years as the recommended minimum for murder, but it can be lower if there are mitigating circumstances.

In his defense, Mr. Pistorius argued that he had shot Ms. Steenkamp by accident, firing four bullets through a bathroom door in the belief that an intruder had entered his upscale villa in Pretoria. Only when he smashed down the door, he said at his trial, did he discover that he had shot Ms. Steenkamp.

Prosecutors said he had killed her in a jealous fury after an argument in the early hours of Feb. 14, 2013.

Mr. Pistorius was not in court on Friday to hear the decision on his sentence.

Ms. Steenkamp’s family said through a spokeswoman that the increased sentence had restored their faith in the judicial system. “This is an emotional thing for them. They just feel that their trust in the justice system has been confirmed this morning,” the spokeswoman, Tania Koen, told Reuters.

Thokozile Matilda Masipa, the High Court judge who presided at Mr. Pistorius’s initial trial, sentenced him to five years for manslaughter in 2014, but prosecutors challenged the verdict, and it was replaced with a finding of murder.

In July 2016, Judge Masipa handed down a six-year term for murder. She explained her seeming leniency by saying that Mr. Pistorius had shown genuine remorse in trying, albeit without success, to apologize in person to Ms. Steenkamp’s family. Many legal analysts had expected a lengthier sentence.

“I am of the view that a long-term imprisonment will not serve justice,” Judge Masipa said at the time. “He’s a fallen hero who has lost his career and is ruined financially.” Ms. Steenkamp died less than a year after the 2012 Olympic Games in London, where Mr. Pistorius carried the South African flag at the closing ceremony.

On Friday, a panel of five judges ruled that Mr. Pistorius should have been sentenced to the 15-year minimum term. In a written ruling, the judges echoed the prosecutors’ assessment that the previous sentence was “shockingly lenient to a point where it has the effect of trivializing this serious offense.”

The ruling said that Mr. Pistorius “displays a lack of remorse and does not appreciate the gravity of his actions.”

It was not clear whether Mr. Pistorius’s lawyers planned an appeal to the Constitutional Court, the country’s highest tribunal.