Østfold [ˈœ̂stfɔɫ] (listen) is a traditional region, a former county and a current electoral district in southeastern Norway. It 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 was Sarpsborg. The county controversially became part of the newly established Viken County on 1 January 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.
The county slogan is "The heartland of Scandinavia". The local dialect is characterized by its geographical proximity to Sweden.