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.