Secretary Island () is an island in southwestern New Zealand, lying entirely within Fiordland National Park. Roughly triangular in shape, it lies between Doubtful Sound in the south and Te Awa-o-Tū / Thompson Sound in the north, with its west coast facing the Tasman Sea. To the east of the island, Pendulo Reach connects Te Awa-o-Tū / Thompson Sound with Doubtful Sound. Steeply sloped, the entirely bush-clad island rises to a chain of several peaks higher than 1000 metres. The highest of these is the Mount Grono, the highest peak in the main New Zealand chain not located in the North or South Island. The island also contains three lakes. The largest, Secretary Lake, over long, is located beneath Mount Grono at an altitude of .