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.

Østfold is among the nation's oldest inhabited regions, with petroglyphs (rock drawings) and burial mounds throughout the area.

The barony of Heggen og Frøland, consisting of the municipalities Askim, Eidsberg and Trøgstad, originally belonged to Akershus - but it was transferred to Østfold in 1768.

Archaeologists also revealed at least seven other previously unknown burial mounds and the remnants of five longhouses with the help of the radar survey.

It is dominated by flat landscape with a lot of woodland in the north and along the Swedish border, a major lake system in the central part, and densely populated lowland area along the coast, with a relatively large archipelago.

The cities of Moss, Sarpsborg, Fredrikstad, and Halden are situated here, along with some relatively highly populated rural municipalities.

They also represent the worship of the Sun in the Bronze Age (depicted in several rock carvings found in the county).