County Sligo (/ˈslaɪɡoʊ/, Irish: Contae Shligigh) is a county in Ireland. It is in the Northern and Western Region and is part of the province of Connacht. Sligo is the administrative capital and largest town in the county. Sligo County Council is the local authority for the county. The population of the county was 70,198 at the 2022 census. It is noted for Benbulben Mountain, one of Ireland's most distinctive natural landmarks.

The county was officially formed in 1585 by Sir Henry Sidney, Lord Deputy of Ireland, but did not come into effect until the chaos of the Nine Years' War ended, in 1603. Its boundaries reflect the Ó Conchobhair Sligigh confederation of Lower Connacht (Irish: Íochtar Connacht) as it was at the time of the Elizabethan conquest.

This confederation consisted of the tuatha, or territories, of Cairbre Drumcliabh, Tír Fhíacrach Múaidhe, Tír Ollíol, Luíghne, Corann and Cúl ó bhFionn. Under the system of surrender and regrant each tuath was subsequently made into an English barony: Carbury, Tireragh, Tirerril, Leyny, Corran and Coolavin. The capital of the newly shired county was placed at Sligo.

A causewayed enclosure discovered in 2003 at Maugheraboy is one of the earliest indications of Neolithic farming activity on the Cúil Irra Peninsula. The nearby megalithic cemetery of Carrowmore forms part of a huge complex of Stone Age remains connecting Carrowkeel in south Sligo to the Ox Mountains, to the Cuil Irra Peninsula, where the passage tomb named after the legendary Queen Maeve, Miosgán Médhbh, dominates the western skyline from the crest of Knocknarea Mountain.

The Caves of Kesh, famous in Irish mythology, are in south County Sligo. A recent decoding of the work of Marinus of Tyre and Ptolemy shows Sligo as the likely location of Nagnata, an important place of assembly in the Iron Age. Famous medieval manuscripts written in the area include the Book of Ballymote, written in the territory of Corran, the Great Book of Lecan, and the Yellow Book of Lecan, both written in Tir Fhiacrach. The patron of the Annals of the Four Masters was Fearghal Ó Gadhra of Coolavin in south County Sligo.

Sligo County Council is the local authority for the county. At the 2019 Sligo County Council election, it was divided into three local electoral areas (LEAs): Ballymote–Tobercurry (7 seats), Sligo–Strandhill (6 seats), and Sligo–Drumcliff (5 seats). Each of these form a municipal district, with the containing Sligo–Strandhill known as the borough district of Sligo.

Under the Local Government (Ireland) Act 1898, County Sligo was divided into the rural districts of Boyle No. 2, Dromore West, Sligo, and Tobercurry, and the urban district of Sligo. Unlike most urban districts, Sligo had retained its borough corporation. The rural districts were abolished in 1925. Sligo Borough Corporation became a borough council in 2002, before being abolished in 2014 in common with all borough and town councils in Ireland.

County Sligo is part of the Dáil constituency of Sligo–Leitrim (4 TDs). At the 2020 election, Martin Kenny (Sinn Féin), Frank Feighan (Fine Gael), Marc MacSharry (Fianna Fáil) and Marian Harkin (Ind) were elected.