The Union of Utrecht (Dutch: Unie van Utrecht) is a treaty signed on January 23, 1579 in Utrecht, the Netherlands, unifying the northern provinces of the Netherlands, until then under control of Spain. The Union of Utrecht is regarded as the foundation of the Republic of the Seven United Netherlands, which was not recognised internationally until the Peace of Westphalia in 1648.
The treaty was signed by Holland, Zeeland, Utrecht, Gelderland and the province (but not the city) of Groningen. The treaty was a reaction of the Protestant provinces to the 1579 Union of Arras (Dutch: Unie van Atrecht), in which the southern provinces (most of them now part of Belgium) declared their support for the Catholic Spanish.
A number of cities, mainly in Flanders and Brabant, joined the Union after it was signed. However, these cities were conquered by the Spanish army, and did not become a part of the Republic.
The rule of Spanish was still recognized by the United Provinces after the Union of Utrecht. However, the Union contributed to the deteriorating relationship between the provinces and their lord, and in 1581 the United Provinces declared their independence of the king in the Oath of Abjuration.