The Protestant Union (German: Protestantische Union), also known as the Evangelical Union, Union of Auhausen, German Union or the Protestant Action Party, was a coalition of Protestant German states. It was formed on 14 May 1608 by Frederick IV, Elector Palatine in order to defend the rights, land and safety of each member. It included both Calvinist and Lutheran states, and dissolved in 1621.

The union was formed following two events. Firstly, the Holy Roman Emperor Rudolf II and Bavarian Duke Maximilian I reestablished Catholicism in Donauwörth in 1607. Secondly, by 1608, a majority of the Imperial Diet had decided that the renewal of the 1555 Peace of Augsburg should be conditional upon the restoration of all church land appropriated since 1552. The Protestant princes met in Auhausen, and formed a coalition of Protestant states under the leadership of Frederick IV on 14 May 1608. In response, the Catholic League organized the following year, headed by Duke Maximilian.Members of the Protestant Union included the Palatinate, Neuburg, Württemberg, Baden-Durlach, Ansbach, Bayreuth, Anhalt, Zweibrücken, Oettingen, Hesse-Kassel, Brandenburg, and the free cities of Ulm, Strasbourg, Nuremberg, Rothenburg, Windsheim, Schweinfurt, Weissenburg, Nördlingen, Schwäbisch Hall, Heilbronn, Memmingen, Kempten, Landau, Worms, Speyer, Aalen and Giengen.However, the Protestant Union was weakened from the start by the non-participation of several powerful German Protestant rulers, notably the Elector of Saxony. The Union was also beset by internal strife between its Lutheran and Calvinist members.In 1619, Frederick V of the Palatinate accepted the crown of Bohemia in opposition to Holy Roman Emperor Ferdinand II. On 3 July 1620 the Protestant Union signed the Treaty of Ulm (German: Ulmer Vertrag), declaring neutrality and declining to support Frederick V. In January 1621, Ferdinand II imposed an imperial ban upon Frederick V and moved his right to elect an emperor to Maximilian. Electoral Palatinate also lost the Upper Palatinate to Bavaria. The Protestant Union met in Heilbronn in February and formally protested Ferdinand's actions. He ignored this complaint and ordered the Protestant Union to disband its army. The members of the union complied with Ferdinand's demand under the Mainz accord in May, and on 14 May 1621 it was formally dissolved.A new separate union without connection to this one emerged twelve years later, the Heilbronn League. It allied some Protestant states in western, central and southern Germany, and fought against the Holy Roman Emperor under the guidance of Sweden and France, which were at the same time parties to that league.