The alleged incident took place at Metropolitan Correctional Center in Chicago, where Kelly is currently being held pending his federal trial for sex crimes.
Citing law enforcement sources, TMZ reports that Kelly was sitting in his bed when a fellow inmate walked into his cell and began attacking him. The inmate was reportedly upset over recent lockdowns at the detention center stemming from large Kelly protests outside the building.
Kelly was examined by a doctor, but did not sustain any serious injuries in the fight, TMZ adds.
Kelly himself is due in court later this fall. He faces federal charges in both Illinois and New York, as well as state charges in Illinois and Minnesota. His alleged crimes range from child pornography and sexual assault, to heading a racketeering scheme designed to supply Kelly with vulnerable, underage girls and later giving them herpes.