This decision comes following his initial five-year conviction for manslaughter was changed to murder last December on appeal.
The South African athlete was taken immediately to jail. Both the prosecution and defence can appeal.
Pistorius, 29, shot Reeva Steenkamp four times through a locked toilet door in February 2013.
He admitted shooting her, but said that he had mistaken Ms Steenkamp for an intruder and acted out of fear.
In an hour-long session, Judge Thokozile Masipa said mitigating circumstances, such as rehabilitation and remorse, outweighed aggravating factors for deviating from the prescribed 15-year sentence for murder.
The six-time Paralympic gold medallist created 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.