The Battle of St. Lucia or the Battle of the Cul de Sac was a naval battle fought off the island of St. Lucia in the West Indies during the American Revolutionary War on 15 December 1778, between the British Royal Navy and the French Navy.

Saint Lucia is an island country of the West Indies in the eastern Caribbean. The island was previously called Iouanalao and later Hewanorra, names given by the native Arawaks and Caribs (respectively), two Amerindian peoples. Part of the Windward Islands of the Lesser Antilles, it is located north/northeast of the island of Saint Vincent, northwest of Barbados and south of Martinique. It covers a land area of 617 km2 with an estimated population of over 180,000 people as of 2018. The nation's capital and largest city is Castries.