Lake Pend Oreille, Mountain lake in Bonner County, United States.

Lake Pend Oreille is a glacially formed lake in northern Idaho's Purcell Trench, 43 miles (69 km) long with a surface area of 148 square miles (380 km2). It reaches 1,152 feet (351 m) at its deepest, the fifth-deepest lake in the United States. The Clark Fork and Pack rivers feed it, and the Pend Oreille River drains it; the Albeni Falls dam, 90 feet (27 m) high, raises its level slightly.

Lake Pend Oreille was shaped by glaciers during the last ice age, and its eastern side is thought to have lain in the path of the Missoula Floods. In 1809 David Thompson set up a North West Company trading post on the shore, and a French-Canadian fur trader in his party is believed to have named the lake. During World War II the south end held Farragut Naval Training Station, the second-largest naval training ground in the world and the largest "city" in Idaho; it is now Farragut State Park.

The name comes from French traders who referred to the Kalispel people, an indigenous group who lived along these shores for centuries. Today, local place names and stories recall this early meeting between European fur traders and the communities who fished and hunted here.

Several ramps and marinas around the lake offer access for boats and watercraft, especially in towns like Sandpoint, Bayview, and Hope. Most facilities rent equipment and have fuel docks, so visitors without their own vessel can get out on the water.

The lake's depth gives it acoustic properties close to the open ocean, with less background noise than ocean test sites, so the Navy's Acoustic Research Detachment tests large-scale submarine prototypes on the Pend Oreille Calibration Station. Sound signatures under study there cannot be monitored by foreign governments in international waters.