The siege of Kreuznach or the Spanish capture of Kreuznach took place on 10 September 1620, at Kreuznach in the Palatinate, where the Army of Flanders, led by the Spanish Don Ambrosio Spinola, defeated the troops of Frederick V, Elector of the Palatinate, during the Palatinate campaign of the Thirty Years' War. The Army of Flanders was a multinational army in the service of the kings of Spain that was based in the Netherlands during the 16th to 18th centuries. Spinola's troops stormed Bad Kreuznach and its garrison surrendered. Later the town was freed on an oath not to rebel against the Holy Roman Empire.

The Palatinate campaign, also known as the Spanish conquest of the Palatinate or the Palatinate phase of the Thirty Years' War was a campaign conducted by the Imperial army of the Holy Roman Empire against the Protestant Union in the Lower Palatinate, during the Thirty Years' War.