The Cologne War (1583–88) devastates the Electorate of Cologne, a historical ecclesiastical principality of the Holy Roman Empire, present-day North-Rhine-Westphalia, in Germany.

The war occurs within the context of the Protestant Reformation in Germany and the subsequent Counter-Reformation, and concurrently with the Dutch Revolt and the French Wars of Religion.Also called the Seneschal's War or the Seneschal Upheaval, and occasionally the Sewer War, the conflict tests the principle of ecclesiastical reservation, which had been included in the religious Peace of Augsburg (1555).

This principle excluded, or "reserved", the ecclesiastical territories of the Holy Roman Empire from the application of cuius regio, eius religio, or "who rules, his religion", as the primary means to determine the religion of a territory.

It stipulated instead that if an ecclesiastical prince converted to Protestantism, he would resign from his position, not force the conversion of his subjects.In December 1582, Gebhard, Truchsess von Waldburg, the Prince-elector of Cologne, had converted to Protestantism.

The principle of ecclesiastical reservation required his resignation.

Instead, he declares religious parity for his subjects and, in 1583, marries Agnes von Mansfeld-Eisleben, intending to convert the ecclesiastical principality into a secular, dynastic duchy.

A faction in the Cathedral Chapter elects another archbishop, Ernst of Bavaria.Initially, troops of the competing archbishops of Cologne fight over control of sections of the territory.

Several of the barons and counts holding territory with feudal obligations to the Elector also hold territory in nearby Dutch provinces, Westphalia, Liege, and the Southern, or Spanish Netherlands.

Complexities of enfeoffment and dynastic appanage expand a localized feud to include supporters from the Electoral Palatinate and Dutch, Scots and English mercenaries on the Protestant side, and Bavarian and papal mercenaries on the Catholic side.

In 1586, it expands further, with direct involvement of Spanish troops and Italian mercenaries for the Catholic side, and financial and diplomatic support from Henry III of France and Elizabeth I of England on the Protestant side.The conflict coincides with the Dutch Revolt, 1568–1648, encouraging participation of the rebellious Dutch provinces and the Spanish.

The Cologne War results in the consolidation of Wittelsbach authority in northwestern German territories and in a Catholic revival on the lower Rhine.

Importantly, it also sets a precedent for outside intervention in German religious and dynastic conflicts.