Asteasu is a town in the province of Gipuzkoa, within the autonomous community of the Basque Country in northern Spain. It lies in the comarca of Tolosaldea, in the inland part of Gipuzkoa, roughly 25 kilometres south of the provincial capital, San Sebastián.
The town sits in hilly terrain typical of the Basque interior, surrounded by green countryside and small rural settlements. Its historic core centres on the parish church of San Pedro, and the municipality retains a traditional character, with agriculture and small-scale industry forming the basis of its local economy.
Asteasu is known in Basque letters as the birthplace of the writer and priest José María Iparragirre (1820–1881), author of the song "Gernikako Arbola," which became an anthem of Basque identity. The town is connected by road to nearby Tolosa and Villabona, and the river Oria flows through the municipality.