Lake Lucerne (German: Vierwaldstättersee, literally "Lake of the four forested settlements" (in English usually translated as forest cantons), French: lac des Quatre-Cantons, Italian: lago dei Quattro Cantoni) is a lake in central Switzerland and the fourth largest in the country.

The lake has a complicated shape, with several sharp bends and four arms. It starts in the south–north bound Reuss Valley between steep cliffs above the Urnersee from Flüelen towards Brunnen to the north before it makes a sharp bend to the west where it continues into the Gersauer Becken. Here is also the deepest point of the lake with 214 m (702 ft). Even more west of it is the Buochser Bucht, but the lake sharply turns north again through the narrow opening between the Unter Nas (lower nose) of the Bürgenstock to the west and the Ober Nas (upper nose) of the Rigi to the east to reach the Vitznauer Bucht. In front of Vitznau below the Rigi the lake turns sharply west again to reach the center of a four-arm cross, called the Chrütztrichter (Cross Funnel). Here converge the Vitznauer Bucht with the Küssnachtersee from the north, the Luzernersee from the west, and the Horwer Bucht and the Stanser Trichter to the south, which is to be found right below the northeast side of the Pilatus and the west side of the Bürgenstock. At the very narrow pass between the east dropper of the Pilatus (called Lopper) and Stansstad the lake reaches its southwestern arm at Alpnachstad on the steep southern foothills of the Pilatus, the Alpnachersee. The lake drains its water into the Reuss in Lucerne from its arm called Luzernersee (which literally translates as Lake of Lucerne).

The entire lake has a total area of 114 km² (44 sq mi) at an elevation of 434 m (1,424 ft) a.s.l., and a maximum depth of 214 m (702 ft). Its volume is 11.8 km³. Much of the shoreline rises steeply into mountains up to 1,500 m above the lake, resulting in many picturesque views including those of the mountains Rigi and Pilatus.

The Reuss enters the lake at Flüelen, in the part called Urnersee (Lake of Uri, in the canton of Uri) and exits at Lucerne. The lake also receives the Muota at Brunnen, the Engelberger Aa at Buochs, and the Sarner Aa at Alpnachstad.

It is possible to circumnavigate the lake by train and road, though the railway route circumvents the lake even on the north side of the Rigi via Arth-Goldau. Since 1980, the A2 motorway leads through the Seelisberg Tunnel in order to reach the route to the Gotthard Pass in just half an hour in Altdorf, Uri right south of the beginning of the lake in Flüelen.

Steamers and other passenger boats ply between the different villages and towns on the lake. It is a popular tourist destination, both for native Swiss and foreigners, and there are many hotels and resorts along the shores. In addition, the meadow of the Rütli, traditional site of the founding of the Swiss Confederation, is on the Urnersee shore. A 35 km commemorative walkway, the Swiss Path, was built around the Lake of Uri to celebrate the country's 700th anniversary in 1991.