Deer Island is a Canadian island located in the Bay of Fundy at the entrance to Passamaquoddy Bay.

At 45 km², the island is the largest in the West Isles Parish which had a 2001 population of 851. Deer Island is entirely rural with the major community being Fairhaven. The economy is primarily driven by the fishing and aquaculture industries although tourism is growing in importance.

The island is famous for playing host to Old Sow, the largest tidal whirlpool in the western hemisphere.

A year-round ferry service connects Deer Island with Letete, New Brunswick. Seasonal ferries operate from Deer Island in the summer months to Eastport, Maine and Campobello Island, New Brunswick.