The Catholic League (Latin: Liga Catholica, German: Katholische Liga) was a coalition of Catholic states of the Holy Roman Empire formed 10 July 1609. While initially formed as a confederation to act politically to negotiate issues vis-à-vis the Protestant Union (formed 1608), modelled on the more intransigent ultra-Catholic French Catholic League (1576), it was subsequently concluded as a military alliance "for the defence of the Catholic religion and peace within the Empire".

Notwithstanding the league's founding, as had the founding of the Protestant Union, it further exacerbated long standing tensions between the Protestant reformers and the adherents of the Catholic Church which thereafter began to get worse with ever more frequent episodes of civil disobedience, repression, and retaliation that would eventually ignite into the first phase of the Thirty Years' War roughly a decade later with the act of rebellion and calculated insult known as the Third Defenestration of Prague on 23 May 1618.

In 1555, the Peace of Augsburg was signed, which confirmed the result of the Diet of Speyer (1526) and ended the violence between the Catholics and the Lutherans in the Holy Roman Empire.

It stated that:

Those occupying a state that had officially chosen either Catholicism or Lutheranism could not practice the religion differing to that of the state.

Although the Peace created a temporary end to hostilities, the underlying basis of the religious conflict remained unsolved. Both parties interpreted it at their convenience, the Lutherans in particular considering it only a momentary agreement. Further, Calvinism spread quickly throughout the Holy Roman Empire, adding a third major Christian worldview to the region, but its position was not supported in any way by the Augsburg terms, since Catholicism and Lutheranism were the only permitted creeds.

The best documented reason of the foundation of the Catholic League was an incident called the Kreuz- und Fahnengefecht [de] (lit. 'Cross and Flag engagement') in Donauwörth, a Free Imperial City within the territory of Bavaria. On 25 April 1606, the Lutheran majority of the town barred the Catholic residents of the town from holding an annual procession on St. Mark's Day, to show the rule of their confession over the town. The Catholics, led by five monks, wanted to pass through the town and on to the nearby village of Ausesheim, showing their flags and singing hymns. They were permitted to do so by the terms of the Peace of Augsburg. The city council would only allow them to re-enter town without flags and singing. The conflict ended in a brawl.

On protest of the bishop of Augsburg, Catholic Emperor Rudolf II of Habsburg threatened an Imperial ban in case of further violation of the rights of the Catholic citizens. Nevertheless, next year similar anti-Catholic incidents of civil disobedience took place, and the participants of the Markus procession were thrown out of town.