The siege of Saint-Omer was a siege in the Thirty Years' War in which a French army under Gaspard III de Coligny, Maréchal de Châtillon, laid siege to the Flemish city of Saint-Omer, defended by a small garrison in command of Lancelot II Schetz, count of Grobbendonck. Despite several initial successes in the capture of the minor forts around Saint-Omer, on the night of 8/9 June a Spanish relief army under Thomas Francis, Prince of Carignano, surprised Châtillon's troops and established a small fort in the middle of the French lines. An entire army corps under Maréchal de La Force was ordered to move towards Saint-Omer to support Châtillon siege, but on 12 July a further Imperial-Spanish force commanded by Ottavio Piccolomini entered Saint-Omer, resolving the French marshals to withdraw.

Ottavio Piccolomini, 1st Duke of Amalfi was an Italian nobleman whose military career included service as a Spanish general and then as a field marshal of the Holy Roman Empire.