Sarpsborg [ˈsɑʂbɔr], historically Borg, is a city and municipality in Østfold county, Norway. The administrative centre of the municipality is the city of Sarpsborg.

Sarpsborg is part of the fifth largest urban area in Norway when paired with neighbouring Fredrikstad. As of 1 January 2016, according to Statistics Norway these two municipalities have a total population of 132,351 with 54,192 in Sarpsborg and 78,159 in Fredrikstad.

Borregaard Industries is, and always has been, the most important industry in the city. The city is also the home of Borg Bryggerier, part of the Hansa Borg Bryggerier, which is Norway's second largest brewery-group.

In Norse times the city was just called Borg (from borg which means "castle"). The background for this was the fortification built by Olav Haraldsson (see History section). Later the genitive case of the name of the waterfall Sarpr (Sarp Falls) was added.

In Norse times Østfold county was called Borgarsýsla which means "the county (sýsla) of Borg" and the law district of southeast Norway was called Borgarþing meaning "the thing/court of Borg".

The old name has been revived in the diocese of Borg (1968) and Borgarting Court of Appeal (1995).

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