The Vajont Dam (or Vaiont Dam) is a disused dam in northern Italy. It is one of the tallest dams in the world, with a height of 262 m (860 feet).

It is situated in the valley of the Vajont River under Monte Toc, in the municipality of Erto e Casso, 100 kilometres (62 mi) north of Venice.

The dam was conceived in the 1920s and eventually built between 1957 and 1960 by Società Adriatica di Elettricità (SADE), at the time the electricity supply and distribution monopoly in northeastern Italy. The engineer was Carlo Semenza (1893–1961). In 1962 the dam was nationalized and came under the control of ENEL as part of the Italian Ministry for Public Works.

On 9 October 1963, during initial filling, a landslide caused a megatsunami in the lake in which 50 million cubic metres of water overtopped the dam in a wave of 250 metres (820 ft), which brought massive flooding and destruction to the Piave Valley below, leading to the complete destruction of several villages and towns, and between 1,900 and 2,500 estimated deaths. The dam itself remained almost intact and two thirds of the water was retained behind it.

This event occurred after ENEL and the Italian government concealed reports and dismissed evidence that Monte Toc, on the southern side of the basin, was geologically unstable. They had disregarded numerous warnings, signs of danger, and negative appraisals. Underestimating the size of the landslide, ENEL's attempt to safely control the landslide by lowering the lake's level came when disaster was almost imminent.