Mýrdalsjökull (pronounced [ˈmirtalsˌjœːkʏtl] ( listen), Icelandic for "(the) mire dale glacier" or "(the) mire valley glacier") is a glacier in the south of Iceland. It is situated to the north of Vík í Mýrdal and to the east of the smaller glacier Eyjafjallajökull. Between these two glaciers is Fimmvörðuháls pass. Its peak reaches 1,493 m (4,898 ft) in height and in 1980 it covered an area of 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 (6.2 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.

As the Eldgjá, a volcanic eruption fissure of about 30 km (19 mi) length, erupting in the year 936, is part of the same volcanic system, it can be regarded as one of the most powerful in the world.^{[citation needed]}

Before the hringvegur (the main 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. 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.