Lake Bienne or Lake Biel or (French: Lac de Bienne, German: Bielersee) is a lake in the west of Switzerland. Together with Lake Morat and Lake Neuchâtel, it is one of the three large lakes in the Jura region of Switzerland. It lies approximately at 47°5′N 7°10′E, at the language boundary between German and French speaking areas.
The lake is 15 km long and up to 4.1 km wide. Its surface area is 39.3 km², the maximum depth 74 m. The lake is located at 429 metres above sea level.
Lake Biel has a catchment area of about 8,305 km². Water remains in the lake for an average of 58 days. The rivers Aare and Zihl/Thielle flowing from Lake Neuchâtel, the Twannbach draining water down from the surplombing first Jura mountain range and the Suze draining water down from the Vallon de St. Imier, are the main tributaries. The Aare was redirected into the lake in 1878, in order to prevent the flooding of the nearby area called "Seeland", and drains the water out of the lake down to Büren through a simultaneously man-made channel. The level of all three lakes is being controlled by a dam built across the channel, in Port.
The largest settlement along the shores of Lake Biel are the bilingual city of Biel/Bienne and its sister city Nidau, both located on the northern end. The St. Peter's Island attracts tourists. The island is located near Erlach/Cerlier. Jean-Jacques Rousseau once spent a few months on the island.
One of the main business in the area is the watch and affiliated microtechnic industries. World famous watch brands have their sites and headquarters in the area, including the Swatch Group and Rolex movements manufacture.
The lake's surface is the lowest point of the canton of Neuchâtel.