Marker is a municipality in Østfold county, Norway. The administrative centre of the municipality is the village of Ørje. Marker was created as a new municipality on 1 January 1964 after the merger of the two old municipalities of Rødenes and Øymark.
The municipality borders on Aremark, Rakkestad, Eidsberg, and Rømskog municipalities in Østfold county, Aurskog-Høland municipality in Akershus county, and on Sweden. European route E18 goes through the municipality.The biggest attractions in the municipality are Ørje Fortress and Basmo Fortress. Basmo Fortress lies on an isolated mountain outcropping between the lakes Rødenessjøen and Hemnessjøen in the northwestern part of the municipality.