Ascain is part of the urban area of Bayonne in the traditional Basque province of Labourd 13 km east of Irun and south of Saint-Jean-de-Luz and the Atlantic Ocean in the western foothills of the Pyrenees. The southern tip of the commune touches the border with Spain at the peak of Larrun mountain. Access to the commune is by the D4 road from Urrugne in the north-west coming into the commune from the west then passing through the town and continuing south-east to Sare. The D918 also comes from Saint-Jean-de-Luz in the north and passes through the north of the commune between two urban areas and continuing east to Saint-Pée-sur-Nivelle. Some 30% of the commune is residential with some small forests in the north and farmland mostly in the south with some farms in the north.