Oscar Pistorius was injured in a prison brawl, less than two weeks after his sentence for the murder of Reeva Steenkamp was more than doubled.
The brawl reportedly broke out when another inmate accused the former Paralympian of spending too long on the prison phone.
Pistorius sustained a bruise following the altercation, a prison department spokesman told the BBC. No other injuries were reported.
"It is alleged that he was involved in an altercation with another inmate over the use of a public phone in the special care unit where both offenders are detained at Attridgeville Correctional Centre," spokesman Singabakho Nxumalo said.
He added that the department had launched an investigation to "establish the facts and to ensure that appropriate action is taken as incidents of assaults are not allowed".
The incident took place just 10 days after South Africa's Supreme Court of Appeal more than doubled Pistorius' prison sentence to 13 years and five months.
Pistorius had previously been sentenced to six years in prison, a term prosecutors called "shockingly lenient".
Women's rights activists said at the time that the short sentence sent out a message of impunity in a country with one of the highest murder rates in the world.
After the Supreme Court ruling, Tania Koen, a spokeswoman for the Steenkamp family, said: "This is an emotional thing for them. They just feel that their trust in the justice system has been confirmed this morning."
Pistorius killed Steenkamp by firing four bullets from a handgun through a closed bathroom door on Valentine's Day in 2014. He claims that he thought Steenkamp was an intruder.