A fire just before midnight on Saturday erupted inside Massy Stores Supermarket at the Castries Waterfront.
According to the Saint Lucia Fire Service, the fire damaged parts of the inside of the supermarket.
They included the cashier area, a change room and a server room, it was reported.
Firefighters battled the blaze for some three hours, a fire service official told St Lucia Times.
They were still on the scene Sunday morning conducting mopping up operations.