Langjökull (Icelandic pronunciation: [ˈlauŋkˌjœːkʏtl̥] ^ⓘ, Icelandic for "long glacier") is the second largest ice cap in Iceland (870 km² (340 sq mi)), after Vatnajökull. It is situated in the west of the Icelandic interior or Highlands of Iceland and can be seen clearly from Haukadalur. It covers the higher parts of the Langjökull volcanic system.

Its volume is 195 km³ (47 cu mi) and the ice is up to 580 m (1,900 ft) thick. The highest point of the ice cap (at Baldjökull [ˈpaltˌjœːkʏtl̥] at the northern end of Langjökull) is about 1,450 m (4,760 ft) above sea level.

In the past, the largest recorded surface area was in 1840.

The glacier is roughly parallel to the direction of the country's active volcanic zone: north-east to south-west. It is about 50 km (31 mi) long and 15 to 20 km (9.3 to 12.4 mi) wide, and has a slightly narrower point roughly between the lake Hvítárvatn on the Kjölur mountain road to the east and the Þrístapajökull glacier to the west, near another smaller glacier, Eiríksjökull, which is not quite connected to Langjökull. It is the nearest large glacier to Reykjavík.

The area of the glacier includes some mountains, e.g. Jarlhettur ([ˈja(r)tl̥ˌhɛhtʏr̥]; "The earl's hat") on the east side of Langjökull, a palagonitic mountain range, which originated in a fissure vent under a glacier during the Ice Age.

The mountain Skríðufell [ˈskriːðʏˌfɛtl̥] (1,235 m (4,052 ft)) is situated on the east, above lake Hvítárvatn. Other mountains on the eastern side of Langjökull are Fjallkirkja [ˈfjatl̥ˌcʰɪr̥ca] (1,177 m (3,862 ft)), Þursaborg [ˈθʏr̥saˌpɔrk] (1,290 m (4,230 ft)) and Péturshorn [ˈpʰjɛːtʏr̥sˌhɔ(r)tn̥] (1,370 m (4,490 ft)).

A little to the east of Fjallkirkja is the hut of the Icelandic Glacier Research Society (also called Jöklarannsóknarfélag in the Jöklarannsóknafélag Íslands [is]), which includes scientists as well as interested amateurs.

A number of outlet glaciers reach down from Langjökull to the valleys and plains below. However various different names exist for these outlet areas and there are outlet areas that do not have glacial names. The outlets include Norðurjökull [ˈnɔrðʏrˌjœːkʏtl̥] and Suðurjökull [ˈsʏːðʏrˌjœːkʏtl̥] to the east, separated by the Skriðufell glacier outlet area; Eystri-Hagafellsjökull (Hagafellsjökull eystri) [ˈeistrɪ-] at the southern end of Langjökull which is separated from Suðurjökull by the Skálpanes and Jarlhettur glacier outlet areas; the mountain Hagafell [ˈhaːɣaˌfɛtl̥] which is also a small outlet area between Eystri-Hagafellsjökull and Vestri-Hagafellsjökull (Hagafellsjökull vestari) [ˈvɛstrɪ-ˌhaːɣaˌfɛlsˌjœːkʏtl̥]; Lónsjökull (Lónjökull) between it and Geitlandsjökull (1,395 m (4,577 ft)) an outpost to the south-west, with its glacier covering a tuya; Svartárjökull, Flosajökull (Flosakarðsjöklar catchment), Þrístapajökull [ˈθriːˌstaːpaˌjœːkʏtl̥] and Baldjökull to the west. From the north east on the far side of the Baldjökull catchment there is Pjófadalajökull, Hengibjörg, Leiðarjökull and Kirkjujökull catchments. The old icecap marginal glacier name of Bláfellsjökull for Suðurjökull is no longer used. Research shows that the outlet glaciers Norðurjökull and Suðurjökull reached as far as lake Hvítárvatn until about 1900 but have retreated rapidly since then.