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 (Peñas de Aya in Spanish).

The name traces back to Oiasso or Oiarso, a Roman town closely connected to the Arditurri mines in the massif of Aiako Harria, which contained large amounts of silver and copper.

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.

The women's side has competed in the highest division of Spanish football, making Oiartzun one of the smallest towns in the country home to a top-flight club.