An investigation is under way after a bomb scare at HMP Full Sutton near Pocklington.
Staff called in specialist police on Thursday because they were concerned a package found at the prison was dangerous.
A Ministry of Justice spokesman said: "A suspect package intercepted by staff at HMP Full Sutton has been deemed safe by specialist police.
"This incident will now be investigated by the prison."
Full Sutton can house more than 600 male adult prisoners in accommodation that has the highest prison security rating in the country.