Fredrikstad , city (1995 pop. 65,148), Østfold co., SE Norway, a port on the Oslofjord (an arm of the Skagerrak) at the mouth of the Glåma River. Manufactures include forest products, processed food, and chemicals. Founded by Frederick II in 1567, it was fortified in the 17th cent. Its 17th-century fortress, once the strongest in Norway and the main depot for the fleet and army, was decommissioned in 1903.