Lake Thun, Lake in the Bernese Oberland, canton of Bern, Switzerland

Lake Thun is an elongated alpine lake in the Bernese Oberland, stretching about 17.5 km east to west and 3.5 km wide, with a maximum depth of 217 m and a surface area of 47.69 km². It lies at 558 m above sea level, six metres below Lake Brienz, and is fed mainly by the Aare and the Kander. The town of Thun sits at its northwestern outlet, while Interlaken lies on the Aare between the two lakes.

Lake Thun and Lake Brienz were once a single body of water called Wendelsee. Around the year 1000, sediment deposits completed the Bödeli, the flat plain between the mountains that now separates the two lakes. Since 1714, the Kander river has also fed Lake Thun, and since 2009 relief tunnels have increased the lake's outflow capacity by 100 m³/s to prevent flooding in Bern.

The lake is named after the town of Thun at its northwestern outlet. Since 1835 passenger ships have linked Thun with Interlaken West; the operator is the railway company BLS AG, and the fleet of ten includes the steamer Blümlisalp, which sails only in summer. Vineyards on the northern shore, in Thun, Spiez, Oberhofen and Hilterfingen, grow Müller-Thurgau, Chardonnay, Garanoir and Pinot Noir, and since 1 January 2008 the "Lake Thun wine region" has held AOC status.

Passenger boats have run on the lake since 1835; the operator is the railway company BLS AG, and the route links Interlaken West station with Thun station via canals dug for the purpose. Ten vessels are in service, among them the steamer Blümlisalp, which sails only in summer. The railway and road run along the southern shore, partly in tunnel.

Between the 1940s and 1960s the Swiss army dumped about 4,600 tonnes of ammunition it judged surplus after the Second World War and unsafe to store into Lake Thun. Similar dumps in other Swiss lakes brought the total to more than 8,000 tonnes. The lake's maximum depth of 217 m kept the material out of sight.