Eidsberg was a municipality in Østfold county, Norway.

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 arms show a bear, which is taken as a symbol for Arnbjørn Jonsson, who lived in Eidsberg.

He was a well-known warrior during the civil war era under King Håkon Håkonsson, until his death in 1240.