The verdict passed by Judge Thokozile Masipa came after an initial five-year conviction for manslaughter was changed to murder last December on appeal.
He was taken immediately to jail. Both the prosecution and defence can appeal, but his lawyers say they will not.
Pistorius had killed his girlfriend at his home in an upscale Pretoria neighborhood in the early hours of Valentine’s Day three years ago.
He, however, claimed the homicide was an accident, after he mistook her for an intruder.
Masipa said the continuing misperception that Pistorius had intentionally killed Steenkamp was something the court had a duty to correct “to prevent unjustified outrage from the public”:
“Our courts are courts of law, not courts of public opinion.”
In an hour-long judgment, she said:
“The life of the accused will never be the same … He is a fallen hero, who has lost his career, and been ruined financially. He cannot be at peace.”
The same judge originally convicted him of manslaughter in 2014 after months of hearings, but a higher court reversed that to murder last year, leading to Wednesday’s sentencing.
Except for an appeal, it will be the final sentencing in a saga that started when Pistorius fired four bullets through his bathroom door on that fateful morning in February 2013.
Meanwhile, many have expressed shock at Pistorius’s sentence, which is much less than the prescribed minimum for murder. South Africans have taken to social media to express their unhappiness, with some describing the prison term as an insult.