Østfold County Municipality (Norwegian: Østfold fylkeskommune) was the regional governing administration of the old Østfold county in Norway. The county municipality was established in its most recent form on 1 January 1976 when the law was changed to allow elected county councils in Norway. The county municipality was dissolved on 1 January 2020, when Østfold was merged with the neighboring counties of Akershus and Buskerud, creating the new Viken county which is led by the Viken County Municipality. The administrative seat is located in Sarpsborg and the county mayor was Ole Haabeth.

The main responsibilities of the county municipality included the running of the 11 upper secondary schools with about 10,000 pupils.^[1] It managed all the county roadways, public transport, dental care, culture, and cultural heritage sites in the county. Public transport was managed through Østfold Kollektivtrafikk.^[2][3]

The Østfold county council (Norwegian: Fylkestinget) was made up of 35 representatives that were elected every four years. The council essentially acted as a Parliament or legislative body for the county and it met several times each year. The council was divided into standing committees and an executive board (fylkesutvalg) which met considerably more often. Both the council and executive board were led by the County Mayor (fylkesordfører) who held the executive powers of the county. The final county mayor was Ole Haabeth.