The man was found dead at about 4:30 a.m. at the jail at the Hall of Justice on Bryant Street, spokeswoman Eileen Hirst said.
He was discovered while breakfast was being distributed to the inmates, Hirst said.
She said she could not release any more details about the incident or the inmate, who police said was 48 years old. His name is being withheld until the medical examiner's office notifies his family of the death.
A joint investigation into the death is being conducted by the sheriff's department, medical examiner's office and the Police Department's homicide unit, Hirst said.