Østfold 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, 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.

Eastern Norway is the geographical region of the south-eastern part of Norway. It consists of the counties Oslo, Akershus, Vestfold, Østfold, Buskerud, Telemark, and Innlandet.