Vestfjorddalen is a valley in Tinn Municipality in Telemark county, Norway.

The 30-kilometre (19 mi) long valley stretches from the lake Tinnsjå westwards past Rjukan, Vemork, and Rjukan Falls to the lake Møsvatn in the west.

The river Måna is dammed in this area, creating the Skardfoss lake.

Just below the dam lies the Rjukan Falls and the Vemork power plant.

After the power plant, the valley remains very narrow, with mountains on both sides reaching 700–800 metres (2,300–2,600 ft) in height.