Castries (listen) is the capital and largest city of Saint Lucia, an island country in the Caribbean. The urban area has a population of approximately 20,000, while the eponymous district has a population of 70,000, as at May 2013. The city stretches over an area of 80 km² (31 sq mi).

Castries is on a flood plain and is built on reclaimed land. It houses the seat of government and the head offices of many foreign and local businesses. The city is laid out in a grid pattern. Its sheltered harbour receives cargo vessels, ferries and cruise ships. It houses duty-free shopping facilities such as Point Seraphine and La Place Carenage. The city is well served by a bus system and taxi service. St Lucia's main post office is in Castries.

Castries is the birthplace of Arthur Lewis, winner of the 1979 Nobel Memorial Prize in Economics, as well as of Derek Walcott, winner of the 1992 Nobel Prize for Literature.