There are several theories about the founding of Haro, though the most realistic theory is that of Domingo Hergueta, who argued that before the town, there was a lighthouse near the village of Cerro de la Mota which illuminated the mouth of the Ebro river. The town received the name of the lighthouse, and in Castilian Spanish evolved into the name 'Haro'.
During the Roman rule of Hispania, a fort called Castrum Bibilium was built in the cliffs of Bibilio.
The first mention of Haro dates back to the year 1040, in a document of king García Sánchez III of Navarre"el de Nájera".
Alfonso VI of León and Castile entrusted the tenencia to Diego López I de Haro after the death of count García Ordóñez and the first of the lords of Biscay to attach the name of this town to his patronymic was Diego's son, Lope Díaz I de Haro
