Before the hearing began, Pistorius was seen hugging his legal team and appeared calm as he sat in the dock and checked his mobile phone.
The last person he embraced before being taken down to the cells was his sister Aimee, who was in tears.
Judge Thozokile Masipa told the packed courtroom in Pretoria that she felt the mitigating circumstances in the case outweighed the aggravating factors - and described Pistorius as a "fallen hero".
She said the fact that the double amputee had tried to save Ms Steenkamp's life following the shooting played a factor in her sentencing, as did his repeated and unsuccessful attempts to apologise to her family.
However, the judge said she gave "very little weight" to the testimony given by Dr Jonathan Scholtz, a psychiatrist who testified for the defence on Pistorius's state of mind, as much of his evidence was "unsupported by anything concrete".
It is possible that the prosecution will appeal on the grounds that the sentence is too lenient.
Sky's Special Correspondent, Alex Crawford, said the initial reaction of Reeva Steenkamp's family was that Judge Masipa's sentence was "fair".