The Gorner Glacier (German: Gornergletscher) is a valley glacier found on the west side of the Monte Rosa massif close to Zermatt in the canton of Valais, Switzerland. It is about 12.4 km (7.7 mi) long (2014) and 1 to 1.5 km (0.62 to 0.93 mi) wide. The entire glacial area of the glacier related to Gorner Glacier is 57 km2 (22 sq mi) (1999), which makes it the second largest glacial system in the Alps after the Aletsch Glacier system; however it ranks only third in length behind the Aletsch and Fiescher Glacier, respectively. Numerous smaller glaciers connect with the Gorner Glacier. Its (former) tributaries are (clockwise on this map): Gornergletscher (after which the whole system is named, but the upper part is almost disconnected from the lower part nowadays), Monte Rosa Gletscher, Grenzgletscher (nowadays by far its main tributary), Zwillingsgletscher, Schwärzegletscher, Breithorngletscher, Triftjigletscher, and Unterer Theodulgletscher (although the last three ones are actually disconnected now).
The Grenzgletscher (English: Border Glacier) between the central Monte Rosa massif and the Liskamm to the south is nowadays by far the lower Gorner Glacier's main tributary. The Gorner Glacier's upper part is almost already disconnected from its lower part. Also the former tributaries Breithorngletscher, Triftjigletscher, and Unterer Theodulgletscher lost their connections to the Gorner Glacier during the last century; the Lower Theodul Glacier became disconnected in the 1980s.