Eidsberg was a municipality in Østfold county, Norway. The administrative centre of the municipality was the town of Mysen. In 2020, Eidsberg was absorbed into the Indre Østfold municipality.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) was named after the old Eidsberg farm (Old Norse: Eiðsberg) because the first church was built here. The first element is the genitive case of eið 'path around a waterfall' and the last element is berg 'mountain'. Prior to 1847, the name was spelled Edsberg.
The coat-of-arms was from modern times. It was 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 during the civil war era under King Håkon Håkonsson, until his death in 1240.