Sarpsborg, 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 2018, according to Statistics Norway these two municipalities have a total population of 136,127 with 55,840 in Sarpsborg and 81,278 in Fredrikstad.

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. Later the genitive case of the name of the waterfall Sarpr (Sarp Falls) was added, it's unclear how Sarpsborg received this part of its name, two interpretations are the most prevalent. The first coming from the Icelandic word Sarpr which means birdcage in English. The other interpretation is that Sarpr means "the one who swallows", probably referring to the local waterfall.

In Norse times Østfold county was called Borgarsysla which means "the count 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).