Lake Balaton (Hungarian: [ˈbɒlɒton]) is a freshwater rift lake in the Transdanubian region of Hungary. It is the largest lake in Central Europe, and one of the region's foremost tourist destinations. The Zala River provides the largest inflow of water to the lake, and the canalized Sió is the only outflow.

The mountainous region of the northern shore is known both for its historic character and as a major wine region, while the flat southern shore is known for its resort towns. Balatonfüred and Hévíz developed early as resorts for the wealthy, but it was not until the late 19th century when landowners, ruined by Phylloxera attacking their grape vines, began building summer homes to rent out to the burgeoning middle class.

In distinction to all other Hungarian endonyms for lakes, which universally bear the suffix -tó 'lake', Lake Balaton is referred to in Hungarian with a definite article; that is, a Balaton 'the Balaton'. It was called lacus Pelsodis or Pelso by the Romans. The name is Indo-European in origin, derived from Slavic *bolto (Czech bláto, Slovak blato, Polish błoto), meaning 'mud, swamp' (from earlier Proto-Slavic boltьno, cf. Slovene: Blatno jezero, Slovak: Blatenské jazero).

In January 846, the Slavic prince Pribina began to build a fortress as his seat of power and several churches in the region of Lake Balaton, in the territory of modern Zalavár surrounded by forests and swamps along the Zala River. His well-fortified castle and capital of the Lower Pannonian Principality became known as Blatnohrad or Moosburg (literally, 'Swamp Fortress'), and it served as a bulwark against both the Bulgarians and the Moravians.

The German name for the lake is Plattensee. It is unlikely it was given that name for being shallow because the adjective platt is a Greek loanword that was borrowed via French and entered general German vocabulary in the 17th century. It is also noteworthy that the average depth of Balaton (3.2 m or 10 ft) is not extraordinary for the area (cf. the average depth of the neighbouring Neusiedler See, which is roughly 1 m or 3.3 ft).

Lake Balaton affects precipitation in the local area. The area receives approximately 5–7 cm (2–3 in) more precipitation than most of Hungary, resulting in more cloudy days and less extreme temperatures. The lake freezes over during winters. The microclimate around Lake Balaton has also made the region ideal for viticulture. The Mediterranean-like climate, combined with the soil (containing volcanic rock), has made the region notable for its production of wines since the Roman period 2,000 years ago.

It always has been an important location, both tactically and culturally(with many folk tales surrounding it). During the Ottoman wars played an important role in defending Royal Hungary where even battles were fought. While a few settlements on Lake Balaton, including Balatonfüred and Hévíz, have long been resort centres for the Hungarian aristocracy, it was only in the late 19th century that the Hungarian middle class began to visit the lake. The construction of railways in 1861 and 1909 increased tourism substantially, but the post-war boom of the 1950s was much larger.

By the turn of the 20th century, Balaton had become a center of research by Hungarian biologists, geologists, hydrologists, and other scientists, leading to the country's first biological research institute being built on its shore in 1927.