Flathead Lake is the largest lake in the western part of the coterminous United States as well as the largest lake in the state of Montana. This lake is one of the cleanest in the world for its size and type. While the Great Salt Lake is technically larger than Flathead Lake, the former is a natural salt lake or inland sea while Flathead Lake is a freshwater lake.

Located in the northwest corner of the state, 7 miles (11 km) south of Kalispell, it is approximately 30 miles (50 km) long and 16 miles (25 km) wide, covering 191.5 square miles (495.9 km˛). Flathead Lake is 30 miles (48 km) southwest of Glacier National Park and is flanked by two scenic highways, which wind along its curving shoreline. On the west side is US Highway 93, and on the east, is Route 35.

The lake is bordered on its eastern shore by the Mission Mountains and on the west by the Salish Mountains. The Flathead valley was formed by the glacial damming of the Flathead River and sustains a remarkably mild climate for a region located this far north.

Once known as "Salish Lake", this body of water takes its name from the Salish (Flathead) Indians who lived at the southern end of the lake on the Flathead Indian Reservation. Kerr Dam, near Polson, regulates the lake's water level and provides hydroelectric power and water for irrigation. The lake has an irregularly-shaped shoreline and several small islands, the largest of which is a state park called Wild Horse Island. These islands cover 5.5 square miles (14.2 square kilometers). Besides the Flathead River, the Swan River is the lake's other major tributary. The lake is inhabited by the native bull trout and cutthroat trout, as well as the non-native lake trout, yellow perch, and lake whitefish. It is also reportedly inhabited by the infamous Flathead Lake Monster.

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