The former Paralympic champion had an altercation with another inmate over a public telephone earlier this month.
In a statement, South Africa's Department of Correctional Services said Pistorius sustained a bruise during the incident inside the special care unit of Atteridgeville Correctional Centre.
Reports suggest fellow prisoners had grown angry with how long the 31-year-old was taking to use the phone, and the fight is now under investigation.
Pistorius is serving a 13-year sentence for killing his girlfriend Reeva Steenkamp on Valentine's Day in 2013.
Last month, his original sentence was more than doubled by the Supreme Court of Appeal after a court ruled it was too lenient.
Double amputee Pistorius was moved to the Atteridgeville Correctional Centre, which caters for inmates with physical disabilities, in November 2016.
Pistorius was originally convicted of culpable homicide - the equivalent of manslaughter over the killing - but this was later upgraded to murder.
He claimed he thought Ms Steenkamp was an intruder when he shot her four times though a toilet door at his home in Pretoria.