The siege of Calais (1940) was a battle for the port of Calais during the Battle of France. The siege was fought at the same time as the Battle of Boulogne, just before Operation Dynamo, the evacuation of the British Expeditionary Force (BEF) through Dunkirk. After the Franco-British counter-attack at the Battle of Arras, German units were held back to be ready to resist a resumption of the counter-attack on 22 May, despite the protests of General Heinz Guderian, the commander of the XIX Armee Korps, who wanted to rush north up the Channel coast to capture Boulogne, Calais and Dunkirk. An attack by part of the XIX Armee Korps was not authorised until 12:40 a.m. on the night of 21/22 May.

Calais is a port city in the Pas-de-Calais department, of which it is a subprefecture. Although Calais is by far the largest city in Pas-de-Calais, the department's prefecture is its third-largest city of Arras. The population of the city proper is 67,544; that of the urban area is 144,625 (2020). Calais overlooks the Strait of Dover, the narrowest point in the English Channel, which is only 34 km (21 mi) wide here, and is the closest French town to England. The White Cliffs of Dover can easily be seen on a clear day from Calais. Calais is a major port for ferries between France and England, and since 1994, the Channel Tunnel has linked nearby Coquelles to Folkestone by rail.