Spokeswoman Linda Foglia says no inmates or workers were injured and the prison in Ossining is secure. Generators are replacing some lost electricity.
Foglia says the fire started at about 3:40 a.m. Monday in the basement of the 185-year-old prison.
The 686 inmates in Block A were evacuated but eventually sent back to their cells. She says the 29 inmates who were in a disciplinary housing unit were also evacuated and have been relocated to another secure area.
Sing Sing houses about 1,730 inmates in Ossining, on the Hudson River 30 miles north of New York City. The phrase "up the river" was coined for it.