Mýrdalsjökull, Icelandic for "(the) mire dale glacier" or "(the) mire valley glacier") is an ice cap in the south of Iceland. It is to the north of Vík í Mýrdal and to the east of the smaller ice cap Eyjafjallajökull. Between these two glaciers is Fimmvörðuháls pass. Its peak reaches 1,493 m (4,898 ft) in height and in the year 1980 it covered an area of approximately 595 km² (230 sq mi).

The icecap of the glacier covers an active volcano called Katla. The caldera of the volcano has a diameter of 10 km (6 mi) and the volcano erupts usually every 40-80 years. The last eruption took place in 1918. Scientists are actively monitoring the volcano, particularly after the eruption of nearby Eyjafjallajökull began in April 2010. Since the year 930, 16 eruptions have been documented.

The Eldgjá, a volcanic eruption fissure about 30 km (19 mi) long, which erupted in the year 936, is part of the same volcanic system.

Before the Hringvegur (the main ring road round the island) was built, people feared traversing the plains in front of the volcano because of the frequent jökulhlaups (glacial floods) and the deep rivers to be crossed, although the road is still vulnerable to major events. Especially dangerous was the glacial flood after the eruption of 1918 when the coastline was extended by 5 km (3.1 mi) by laharic flood deposits.

Mýrdalsjökull is an exceedingly wet location, with models suggesting it receives more than 10 metres of precipitation annually.