Sarpsborg (särpsˈbôr) [key], city (1995 pop. 46,474), Ostfold co., SE Norway, a port on the Glåma River near its mouth in the Oslofjord. Manufactures include forest products, chemicals, textiles, and electrical products. There is a large hydroelectric plant. Sarpsborg was founded in 1016 by Olaf II, was burned by the Swedes in 1567, and was rebuilt in 1839. Skjeberg Church, a medieval stone structure, is nearby.