Judge Thokozile Masipa told the High Court in Pretoria that giving the 27-year-old a non-custodial sentence would send "the wrong message to the community", but that a severe punishment "would not be appropriate as it lacks the element of mercy".
Pistorius was found guilty of culpable homicide (manslaughter), which carries a maximum sentence of 15 years behind bars, but was cleared of murder after claiming that he mistook his girlfriend for an intruder when he fired four gunshots through a toilet door at his home in the early hours of Valentine's Day last year.
The athlete's defence wanted the judge to impose a three-year house arrest, which included a community service order, while the prosecution called for a 10-year sentence in jail.
Pistorius was also sentenced to three years behind bars after being found guilty of a firearms offence, but the punishment has been suspended for five years.
Pistorius began his murder trial seven months ago.