Fredrikstad was built on the site where Sarpsborg earlier stood (but which was rebuilt about 15km upstream after it burned down). It was founded in 1567 by king Frederik II.
The town centre is situated on the west bank of Glomma, while the old town on the east bank is northern Europe's best preserved medieval fortified town.
Fredrikstad used to have a large sawmill industry and was an important harbour for timber export, then later on shipbuilding, until the main yard was closed in the 1980s. The main industries are currently various chemical plants and other light industry.