According to the Mirror, The six-time paralympic champion, who had his murder sentence more than doubled last month, was queuing to use the phone at Atteridgeville Correctional Centre, near Pretoria, when he reportedly began fighting with an inmate "he knows well".
The 31-year-old, who is serving a 13-year jail sentence for murdering his girlfriend Reeva Steenkamp, sustained a bruised arm from the incident.
Singabakho Nxumalo, from the Department of Correctional Services, said, "Oscar Pistorius sustained a bruise following an alleged incident with another inmate over the use of a public phone in the special care unit where both offenders are detained at Attridgeville Correctional Centre."
Nxumalo claimed that an official investigation into the incident had been launched.
Pistorius had his sentence for the 2013 murder of his girlfriend Reeva Steenkamp, 29, increased to 13 years last month in a successful appeal by prosecutors.
Pistorius was initially imprisoned for six years last July after being found guilty on appeal of murdering Steenkamp on Valentine's Day 2013 in a case that attracted worldwide interest.