Østfold (Norwegian pronunciation: [ˈœ̂stfɔɫ] ) is a county in Eastern Norway, which from 1 January 2020 to 31 December 2023 was part of Viken. Østfold borders Akershus and southwestern Sweden (Västra Götaland County and Värmland), while Buskerud and Vestfold are on the other side of Oslofjord. The county's administrative seat is Sarpsborg. The county controversially became part of the newly established Viken County on 1 January 2020. On 1 January 2024, Østfold was re-established as an independent county, however without the former municipality of Rømskog, which was amalgamated with the Akershus municipality Aurskog-Høland in 2020.
Many manufacturing facilities are situated here, such as the world's most advanced biorefinery, Borregaard in Sarpsborg. Fredrikstad has shipyards. There are granite mines in Østfold and stone from these were used by Gustav Vigeland.