"The incident involved multiple inmate on inmate altercations in three housing units", the department said.
They say the incident resulted in 17 inmates requiring outside medical attention and seven inmates were killed.
State Law Enforcement Division agents helped secure the Lee Correctional Institution about 3 a.m. Monday after the fights began at 7:15 p.m. Sunday, prisons spokesman Jeff Taillon said.
No officers were injured, said Jeffrey Taillon, communications director for the South Carolina Department of Corrections.
SC state police have been called to an "ongoing situation" at one of the state's most unsafe prisons.
Some local media outlets reported the local coroner had been called to the scene.
Lee is a maximum-security prison that houses 1,600 inmates, including some of the state's most violent offenders, reports the State.
The State adds that one inmate died in fights there in both 2017 and 2016 - and that five inmates were injured in a separate fight in 2016.