Lough Erne (/lɒx ɜːrn/ LOKH URN, from Irish Loch Éirne) is two connected lakes in County Fermanagh, Northern Ireland. It is the second-largest lake system in Northern Ireland and Ulster, and the fourth largest in Ireland. The lakes are widened sections of the River Erne, which flows north and then curves west into the Atlantic. The smaller southern lake is called the Upper Lough as it is higher up the river. The larger northern lake is called the Lower Lough or Broad Lough. The town of Enniskillen lies on the short stretch of river between the lakes. The lake has more than 150 islands, along with many coves and inlets. The River Erne is 80 miles (129 km) long and drains an area of about 1,680 square miles (4,350 km²).

Lough Erne (Irish: Loch Éirne) appears to be named after an ancient population group called the Érainn, or after a goddess from which the Érainn took their name. Since tribes were often named after a divine ancestor, T. F. O'Rahilly suggested that the Érainn took their name from a goddess named Érann and that Loch Éirne probably means 'lake of (the goddess) Érann'. O'Rahilly and other scholars have connected these names to Ériu (modern Éire), the goddess after which Ireland is named. He writes that the earlier forms of these goddess names were Everna/Iverna and Everiu/Iveriu and that both come from "the Indo-European root ei-, implying motion". In his view Érann and Ériu would thus appear to mean "she who travels regularly", explained as "the sun-goddess, for the sun was the great celestial Traveller". Alternatively, John T. Koch suggests that Ériu was a mother goddess whose name comes from an Indo-European word stem meaning "fat, rich, fertile".

In Irish mythology and folklore, there are three tales about the lake's origins. One says that it is named after a mythical woman named Erne, Queen Méabh's lady-in-waiting at Cruachan. Erne and her maidens were frightened away from Cruachan when a fearsome giant emerged from the cave of Oweynagat. They fled northward and drowned in a river or lake, their bodies dissolving to become Lough Erne. Patricia Monaghan notes that "The drowning of a goddess in a river is common in Irish mythology and typically represents the dissolving of her divine power into the water, which then gives life to the land". Another tale says that it was formed when a magical spring-well overflowed, similar to the tale of Lough Neagh. The third says that, during a battle between the Érainn and the army of High King Fíachu Labrainne, it burst from the ground and drowned the Érainn. In Cath Maige Tuired ("the Battle of Moytura"), it is listed as one of the twelve chief loughs of Ireland. Historically, the lake was also called Loch Saimer (Samhaoir). In folklore Partholón killed his wife's favourite hound, Saimer, in a fit of jealous rage, and the lake was named after it.

Lough Erne is the setting of a folk tale known as "The Story of Conn-eda" or "The Golden Apples of Lough Erne", which appears in Fairy and Folk Tales of the Irish Peasantry (1888). In the tale, Conn-eda goes on a quest to procure three golden apples, a black steed and a supernatural hound from a city underneath Lough Erne. The city is ruled by a king of the Fir Bolg.

The Menapii are the only known Celtic tribe specifically named on Ptolemy’s AD 150 map of Ireland, where they located their first colony, Menapia, on the Leinster coast circa 216 BC. They later settled around Lough Erne, becoming known as the Fir Manach, and giving their name to Fermanagh and Monaghan. Mongán mac Fiachnai, a 7th-century King of Ulster, is the protagonist of several legends linking him with Manannán mac Lir. They spread across Ireland, evolving into historic Irish (also Scottish and Manx) clans.

The Annals of Ulster were written in the late 15th century on Belle Isle, an island in Upper Lough Erne.

During the Second World War, RAF Castle Archdale was based on Lough Erne, providing an essential airbase for the Battle of the Atlantic and the battle against U-boats. A secret agreement with the Irish Government permitted flying boats based there to fly West straight across neutral Ireland to the Atlantic, avoiding the two-hour detour that would have been necessary for aeroplanes based in Northern Ireland. This flight path became known as 'The Donegal Corridor'. An example of the many ways Ireland assisted the allies while remaining neutral.

In November 2012, it was announced that the Lough Erne Resort, a hotel on the southern shore of the Lower Lough, would host the 39th G8 summit.