The Aletsch Glacier (German: Aletschgletscher) or Great Aletsch Glacier (Grosser Aletschgletscher) is the largest glacier in the Alps. It has a length of about 23 km (14 mi) (2014), has about a volume of 15.4 km³ (3.7 cu mi) (2011), and covers about 81.7 km² (31.5 square miles) (2011) in the eastern Bernese Alps in the Swiss canton of Valais. The Aletsch Glacier is composed of four smaller glaciers converging at Concordia Place, where its thickness was measured by the ETH to be still near 1 km (3,300 ft). It then continues towards the Rhône valley before giving birth to the Massa. The Aletsch Glacier is – like most glaciers in the world today – a retreating glacier. As of 2016, since 1980 it lost 1.3 kilometres (0.81 mi) of its length, since 1870 3.2 kilometres (2.0 mi), and lost also more than 300 metres (980 ft) of its thickness.

The whole area, including other glaciers is part of the Jungfrau-Aletsch Protected Area, which was declared a UNESCO World Heritage Site in 2001.