Prosecutors had argued that the six-year term for murdering Reeva Steenkamp was “shockingly light”.
Ms Steenkamp’s parents were “emotional” as they watched the ruling at home on TV, a spokeswoman said.
“They feel there has been justice for Reeva. She can now rest in peace,” Tania Koen told Associated Press.
“But at the same time, people think this is the end of the road for them… the fact is they still live with Reeva’s loss every day,” Ms Koen said.
Oscar Pistorius claimed he shot dead Ms Steenkamp on Valentine’s Day in 2013 after mistaking her for a burglar.
The Supreme Court of Appeal in Bloemfontein has now given Pistorius the minimum 15 years prescribed for murder in South Africa, less time already served.
The lower court had justified the six-year sentence by citing mitigating circumstances such as rehabilitation and remorse. It said they outweighed aggravating factors such as his failure to fire a warning shot.