Doubtful Fjord (Doubtful Sound) is a large fjord in southwestern New Zealand, in the same region as the smaller but better known Milford Sound. It is home to an unusually large population of bottlenose dolphins, as well as a variety of birds, fur seals, penguins, and often even humpback whales.

Unlike Milford Sound, Doubtful Sound is inaccessible by road, so boat cruises are the only transportation available to visitors. Day trips operate out of the small nearby town, Manapouri.