Pistorius was taken to hospital on Thursday after falling ill and was kept at the medical facility overnight for observation.
"Oscar has been discharged and is back in prison," correctional services department spokesman Logan Maistry said.
Prison officials have not disclosed details about Pistorius's illness, but local media reports said he suffered chest pains and prison staff suspected a heart attack.
News that makes sense
Your trusted source for staying up-to-date with the world around you. Get free daily news updates and analysis, straight to your inbox.
Last August, Pistorius denied trying to kill himself after he was treated in hospital for wrist injuries.
The athlete was sentenced to six years in prison in July last year after being found guilty of murdering his girlfriend, Reeva Steenkamp, on Valentine's Day 2013.
(Reporting by Olivia Kumwenda-Mtambo; Editing by Toby Davis)
