A spokesman for Ms Steenkamp's family said the ruling "verified there was justice".
Pistorius's brother Carl said on social media that he was "shattered. Heartbroken. Gutted".
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 him 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.
The athlete, 31, was not in court to hear the decision.
He was initially given a five-year term for manslaughter in 2014, but was found guilty of murder on appeal in 2015.
Pistorius shot Ms Steenkamp four times through a locked toilet door at his home in the capital Pretoria.
Previously, the six-time Paralympic gold medallist had made history by becoming the first amputee sprinter to compete at the Olympics, in 2012 in London, running on prosthetic "blades".
He had his legs amputated below the knee as a baby.
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