The siege of Leuven was an important siege in the Thirty Years' War in which a Franco-Dutch army under Frederick Henry of Orange and the French Marshals Urbain de Maillé-Brezé and Gaspard III de Coligny, who had invaded the Spanish Netherlands from two sides, laid siege to the city of Leuven, defended by a force of 4,000 comprising local citizen and student militias with Walloons, Germans, Spanish and Irish of the Army of Flanders under Anthonie Schetz, Baron of Grobbendonck. Poor organization and logistics and the spread of sickness among the French, along with the appearance of an Imperial-Spanish relief army of 11,000 under Cardinal-Infante Ferdinand and Ottavio Piccolomini, forced the invading army to lift the siege. This failure allowed the Spanish forces to take the initiative and soon the invaders were forced into a headlong retreat.

Leuven or Louvain is the capital and largest city of the province of Flemish Brabant in the Flemish Region of Belgium. It is located about 25 kilometres east of Brussels. The municipality itself comprises the sub-municipalities of Heverlee, Kessel-Lo, Leuven proper, Wilsele, Wijgmaal and part of Haasrode and Korbeek-Lo. It is the eighth largest city in Belgium, with more than 100,244 inhabitants.