Aire-sur-la-Lys (French: [ɛːʁ syʁ la lis]; West Flemish: Ariën-aan-de-Leie; literally "Aire on the Lys") is a commune in the Pas-de-Calais department in northern France.

[3] The commune is located 16 kilometres (10 mi) southeast of Saint-Omer, at the junction of the N43 with several departmental roads, by the banks of the Leie (French: Lys) and the Laquette rivers.

It developed around a fort or castrum built by Baldwin II, Count of Flanders in response to the Norman invasions.

It is commonly known as Le Bailliage (The Bailiwick), because at various times between 1634 and 1789, it served as the seat of the court bailiffs.

In 1595, the mayor, one Jacques de Caverel had obtained permission from Brussels to raise taxes on beer and wine for the construction of the guardhouse on the city marketplace and the monument was inaugurated on 22 November 1600.

After the return of Aire to the kingdom of France by the Treaty of Utrecht in 1713, King Louis XIV gave permission to build a new and more prestigious Town Hall, in recognition of the suffering of the city during the siege of 1710 under Dutch occupation.