The Siege of Saint-Omer (May 24 – July 16, 1638) 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 Schetz. 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 July 12 a further Imperial-Spanish force commanded by Ottavio Piccolomini entered Saint-Omer, resolving the French marshals to withdraw.