Carcastillo is a town and municipality in the province and autonomous community of Navarre, in northern Spain. It lies about 70 kilometres from Pamplona, the regional capital.
Until the middle of the 19th century, the town belonged to the abbey of Santa María de la Oliva, a Cistercian monastery located roughly two kilometres from Carcastillo.
The town stands on the banks of the river Aragón, to the north of the Bardenas Reales.