The Aletsch Glacier (German: Aletschgletscher) or Great Aletsch Glacier (German: Grosser Aletschgletscher) is the largest glacier in the Alps. It has a length of about 23 km (14 mi) and covers more than 120 square kilometres (46 sq mi) in the eastern Bernese Alps in the Swiss canton of Valais. The Aletsch Glacier is composed by three smaller glaciers converging at Concordia, where its thickness is estimated to be near 1 km (3,300 ft). It then continues towards the Rhone valley before giving birth to the Massa River.

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

The Aletsch Glacier is one of the many glaciers located between the cantons of Berne and Valais on the Bernese Alps located east of the Gemmi Pass. The whole area is considered to be the largest glaciated area in western Eurasia. The Fiescher and Aar glaciers lying on the east have similar extensions.

Except the Finsteraarhorn, all the highest summits of the Bernese Alps are located within the drainage basin of the glacier. The Jungfrau and Mönch constitute the northern boundary; the Gross Fiescherhorn and Gross Wannenhorn lie on its east side; finally the culminating point, the Aletschhorn (4,195 meters/13,763 feet) is located on the west side.