The Union of Utrecht (Dutch: Unie van Utrecht) was a treaty signed on 23 January 1579 in Utrecht, Netherlands, unifying the northern provinces of the Netherlands, until then under the control of Habsburg Spain.

The Union of Utrecht is regarded as the foundation of the Republic of the Seven United Provinces, which was not recognized by the Spanish Empire until the Twelve Years' Truce in 1609.

The treaty was signed on 23 January by Holland, Zeeland, Utrecht (but not all of Utrecht), 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 two southern provinces and a city declared their support for Roman Catholic Spain.

During the following months of 1579, other states signed the treaty as well, such as Ghent, cities from Friesland, as well as three of the quarters of Guelders (Nijmegen Quarter, Veluwe Quarter, Zutphen County). In the summer of 1579, Amersfoort from the province of Utrecht also joined, together with Ypres, Antwerp, Breda and Brussels. In February 1580, Lier, Bruges and the surrounding area also signed the Union. The city of Groningen shifted in favor under influence of the stadtholder for Friesland, George van Rennenberg, and also signed the treaty. The fourth quarter of Guelders, Upper Guelders, never signed the treaty. In April 1580, Overijssel and Drenthe signed on.

This leads to a general and simplified overview of the parts that joined:

the County of Holland

the County of Zeeland

the Lordship of Utrecht

the Duchy of Guelders

the Lordship of Groningen

the Lordship of Friesland

the County of Drenthe

the Lordship of Overijssel

the Duchy of Brabant

the County of Flanders

the cities of Tournai and ValenciennesAntwerp was the capital of the union until its fall to the Spanish.Flanders was almost entirely conquered by the Spanish troops, as was half of Brabant. The United Provinces still recognized Spanish rule after the Union of Utrecht. However, the Union contributed to the deterioration in the relationship between the provinces and their lord, and in 1581 the United Provinces declared their independence of the king in the Act of Abjuration.

The Twelve Years' Truce of 1609 marked a pause in one of history's longest running conflicts, the Eighty Years' War, effectively acknowledging Dutch independence. As Pieter Geyl puts it, the truce marked "an astonishing victory for the Dutch." They gave up no land and did not agree to halt their attacks on Spanish colonies and the Spanish trade empire. In return the Spanish granted the United Provinces de facto independence by describing them as "Free lands, provinces and states against who they make no claim" for the duration of the truce.