Oiartzun (Basque: Oiartzun, Spanish: Oyarzun) is a town and municipality located in the Basque Country, in the province of Gipuzkoa lying at the foot of the massif Aiako Harria.

The name traces back to the Oiasso or Oiarso of the Roman period, an important town dedicated to mining and marine activities. However, it has been pinpointed actually in the current border town of Irun, so the name may have referred to the whole area.

A river bearing the same name meanders through the meadows and neighbourhoods of this sparse municipality. That is actually one of its main features, it is scattered in different quarters across the valley. Elizalde lies on a prominence and it is the central nucleus of the town, with the main church San Esteban and town hall being located there. At the foot of Elizalde lies Altzibar by the Oiartzun River. Ugaldetxo developed into an urban built-up area surrounded by industrial estates near the AP-8 motorway, while the rest of the neighbourhoods have hung onto their rural and picturesque landscape to a large extent.

Alfonso VIII of Castile chartered it as town but it took some decades before its independence from Orereta was total.