Fossby is a town in Aremark Municipality, in Østfold, Norway. Fossby lies at 119 m above sea level. It has 318 inhabitants. The nearest city is Halden, 21 km to the west. Among the landmarks around are Fredriksten (21 km), Rød herregård (22 km) and Immanuel Church (21 km). Among the natural features are Sarp Falls (33 km) and Galteryggen (16.2 km).