In the Middle Ages, Haro grew and absorbed the population of nearby towns such as Tondón and Bilibio. In 1187, Alfonso VIII of Castile gave the town its own fuero legal code, confirmed by Alfonso X the Wise (1254), which dictated the political, administrative, criminal and economic system to which the town would be subject. In the late Middle Ages (1358), a meeting was held in Haro to regulate a brotherhood of different towns in order to defend them against the abuses of the time.
In the fourteenth and fifteenth centuries, the town passed into the hands of the Trastamara family and subsequently Navarre, until John II of Castile gave it to Don Pedro Fernández de Velasco in 1430, along with the title of Count of Haro, for his help in the struggle against John II of Aragon.
In 1627, a printing press was set up in Haro and the first published work was printed by Juan de Mongastón in 1631.