Fredrikstad is a city and municipality in Østfold county, Norway. The administrative center of the municipality is the city of Fredrikstad. The city of Fredrikstad was founded in 1567 by King Frederick II, and established as a municipality on 1 January 1838 (see formannskapsdistrikt). The rural municipality of Glemmen was merged with Fredrikstad on 1 January 1964. The rural municipalities of Borge, Onsøy, Kråkerøy, and Rolvsøy were merged with Fredrikstad on 1 January 1994.
The city straddles the river Glomma where it meets the Skagerrak. Along with neighboring Sarpsborg, Fredrikstad forms the fifth largest city in Norway: Fredrikstad/Sarpsborg. As of 1 January 2017, according to Statistics Norway, these two municipalities have a total population of 135,347 with 80,207 in Fredrikstad and 55,140 in Sarpsborg. Fredrikstad was built at the mouth of Glomma as a replacement after Sarpsborg (15 kilometers (9 miles) upstream) was burnt down by the Swedish Army in the 1500s. Some of the citizens stayed behind and rebuilt their old town at its original site and got their city status back in 1839. The city center is on the west bank of the Glomma, while the old town on the east bank is Northern Europe's best preserved fortified town. Fredrikstad used to have a significant sawmill industry and was an important harbor for timber export, then later on shipbuilding, until the principal yard was closed in 1988. The main sectors are currently various chemical plants and other light industry. In 2005, Fredrikstad was the final host port for the Tall Ships' Race, attracting thousands to the city.