Pistorius, who fatally shot his girlfriend four times through a toilet door, has already served 12 months in prison for her death.
But the original manslaughter conviction was increased to murder by the Supreme Court of Appeal in December.
Judge Thokozile Masipa, whose sentence at the initial manslaughter trial was criticised by women’s groups as too lenient, said she had accepted the defence argument that a lesser punishment was appropriate.
“Public opinion may be loud and persistent but it can play no role in the decision of this court,” Masipa said on Wednesday. “I am of the view that a long term in prison will not serve justice.”
Pistorius, who stood impassively as the sentence was read out, hugged members of his legal team and chatted briefly with his sister Aimee before being led away by police.
Women’s rights groups say Pistorius has received preferential treatment by the justice system compared to non-whites and those without his wealth or international celebrity status. His backers say he did not intend to kill Steenkamp.
The state and large sections of the South African public had demanded a 15-year jail term, the prescribed minimum term for murder, saying he had shown no remorse for the 2013 killing.
It was unclear whether the state, which has two weeks to appeal, would accept Wednesday’s sentence.
Pistorius’ defence team said he would not appeal and that their client would be able to apply for parole after serving half to two-thirds of the sentence.
Steenkamp’s parents, Barry and June, were present in the courtroom, which was packed with relatives of both Pistorius and Steenkamp, journalists and other observers.
“The family accepts the judgment,” Anneliese Burgess, a spokeswoman for the Pistorius family, said outside the courthouse.
The Steenkamp family did not criticise the sentence. “The family has said it before they wanted the law to run its course. It has done so. No further comment. They will keep a dignified silence,” said Dup de Bruyn, the Steenkamp family representative.
Legal expert Marius du Toit described the sentence as “lenient but not wrong”.
Some people who gathered outside the courthouse were critical.
“The law didn’t take its course,” said Dukes Masanabo, a South African sports official who had hoped Pistorius would be sentenced to 10 to 12 years, not six.
He said the sentence was too light because Pistorius was sentenced to almost the same sentence – five years – for his earlier manslaughter conviction.
The track star, who had the lower part of his legs amputated when he was a baby, was freed from prison last October after almost a year behind bars.
He was to serve the remainder of his five-year term under house arrest at his uncle’s house in a wealthy suburb of the capital. Pistorius has been living with his uncle since.
In her ruling on Wednesday, Masipa said that although the Steenkamps had suffered a great loss, Pistorius’s life and career were also in ruins.
“The life of the accused shall also never be the same. He is a fallen hero and can never be at peace,” she said.
The judge agreed with defence that the Pistorius who shot Steenkamp in the early hours of St Valentine’s Day was not the gold-medal-winning athlete but a vulnerable 1.5m tall man.
She said that there was no indication at all that the deceased was in an abusive relationship with Pistorius.
She also said there was no evidence there had been a row between Pistorius and Steenkamp before her death, as suggested by the prosecution.
Pistorius says he fired four shots into the toilet door at his luxury Pretoria home in the mistaken belief that an intruder was hiding behind it.
His defence argued that his disability and mental stress that occurred in the aftermath of the killing should be considered as mitigating circumstances to reduce his sentence.
Legal analysts were divided by the ruling in a country beset by high levels of violent crime against women.
“To reduce from 15 to 6 years in the circumstances of the case seems to me to be unduly generous to Oscar,” Paul Hoffman, a lawyer and director of rights group Accountability Now, said.
“It’s quite possible that having invested so much effort in the prosecution … that (state prosecutor) Gerrie Nel will saddle up again and ride out in an effort to get a bigger sentence,” he said.
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