Seneca Lake is the largest of the glacial Finger Lakes of the U.S. state of New York, and the deepest lake entirely within in the state. It is promoted as being the lake trout capital of the world, and is host of the National Lake Trout Derby. Because of its depth, Seneca Lake has been a testing site for submarines. The lake takes its name from the Seneca nation of Native Americans.

At 38 miles (61 km) long, It is the second longest of the Finger Lakes and has the largest volume, estimated at 4.2 trillion US gallons (16 km³), roughly half of the water in all the Finger Lakes. It has a maximum depth of 618 feet (188 m), and a mean depth of 291 feet (89 m). It has a surface area of 42,800 acres (173 km2).

It is fed by underground springs and replenished at a rate of 328,000 gallons (29,520 m³) per minute. These springs keep the water moving in a constant circular motion, giving it little chance to freeze over. Because of Seneca Lake's great depth, it remains a constant 39.2 °F (4.0 °C). During the summer months however, the top 10 to 15 feet (3.0 to 4.6 m) does warm up to a pleasant 70–80 °F (21–27 °C).