Eidsberg is a municipality in Østfold county, Norway. The administrative centre of the municipality is the town of Mysen. Neighboring municipalities are Rakkestad, Askim, Skiptvet, Trøgstad, and Marker in Østfold, and Aurskog-Høland in Akershus.
Eidsberg was established as a municipality on 1 January 1838 (see formannskapsdistrikt). The town of Mysen was separated from Eidsberg to form a municipality of its own on 1 July 1920, but it was merged back into the municipality of Eidsberg on 1 January 1961.
The municipality (originally the parish) is named after the old Eidsberg farm (Old Norse: Eiðsberg), since the first church was built here. The first element is the genitive case of eið which means "path around a waterfall" and the last element is berg which means "mountain". Prior to 1847, the name was spelled "Edsberg".
The coat-of-arms is from modern times. They were granted on 16 March 1962. The arms show a bear, which is taken as a symbol for Arnbjørn Jonsson, who lived in Eidsberg. The bear is canting for Bjørn (Norwegian: bear). He was a well known warrior in during the civil war era under King Håkon Håkonsson, until his death in 1240.