The Outer Hebrides () or Western Isles (Scottish Gaelic: Na h-Eileanan Siar [nə ˈhelanən ˈʃiəɾ] (listen) or Na h-Eileanan an Iar [nə ˈhelanən əˈɲiəɾ] (listen) or Na h-Innse Gall ("islands of the strangers"); Scots: Waster Isles), sometimes known as the Long Isle/Long Island (Scottish Gaelic: An t-Eilean Fada), is an island chain off the west coast of mainland Scotland. The islands are geographically coextensive with Comhairle nan Eilean Siar, one of the 32 unitary council areas of Scotland. They form part of the archipelago of the Hebrides, separated from the Scottish mainland and from the Inner Hebrides by the waters of the Minch, the Little Minch, and the Sea of the Hebrides. Scottish Gaelic is the predominant spoken language, although in a few areas English speakers form a majority.

Most of the islands have a bedrock formed from ancient metamorphic rocks, and the climate is mild and oceanic. The 15 inhabited islands have a total population of 26,830 and there are more than 50 substantial uninhabited islands. The distance from Barra Head to the Butt of Lewis is roughly 210 kilometres (130 mi).

There are various important prehistoric structures, many of which pre-date the first written references to the islands by Roman and Greek authors. The Western Isles became part of the Norse kingdom of the Suðreyjar, which lasted for over 400 years, until sovereignty over the Outer Hebrides was transferred to Scotland by the Treaty of Perth in 1266. Control of the islands was then held by clan chiefs, principal amongst whom were the MacLeods, MacDonalds, Mackenzies and MacNeils. The Highland Clearances of the 19th century had a devastating effect on many communities, and it is only in recent years that population levels have ceased to decline. Much of the land is now under local control, and commercial activity is based on tourism, crofting, fishing, and weaving.

Sea transport is crucial, and a variety of ferry services operate between the islands and to mainland Scotland. Modern navigation systems now minimise the dangers, but in the past the stormy seas have claimed many ships. Religion, music and sport are important aspects of local culture, and there are numerous designated conservation areas to protect the natural environment.