Sligo (/ˈslaɪɡoʊ/; Irish: Sligeach [ˈʃl̠ʲɪɟəx]; lit. 'abounding in shells') is a coastal seaport and the county town of County Sligo, Ireland, within the western province of Connacht. With a population of 20,608 in 2022, it is the county's largest urban centre (constituting 29.5% of the county's population) and the 24th largest in the Republic of Ireland.

Sligo is a commercial and cultural centre situated on the west coast of Ireland. Its surrounding coast and countryside, as well as its connections to the poet W. B. Yeats, have made it a tourist destination.

Sligo is the anglicisation of the Irish name Sligeach, meaning "abounding in shells" or "shelly place". It refers to the abundance of shellfish found in the river and its estuary, and from the extensive shell middens in the vicinity. The river now known as the Garavogue (Irish: An Gharbhóg), perhaps meaning "little torrent", was originally called the Sligeach. It is listed as one of the seven "royal rivers" of Ireland in the ninth century AD tale The Destruction of Da Dergas Hostel. The river Slicech is also referenced in the Annals of Ulster in 1188.

The Ordnance Survey letters of 1836 state that "cart loads of shells were found underground in many places within the town where houses now stand". The whole area, from the river estuary at Sligo, around the coast to the river at Ballysadare Bay, is rich in marine resources which were utilised as far back as the Mesolithic period.

The importance of Sligo's location in prehistory is demonstrated by the abundance of ancient sites close by and even within the town. For example, Sligo town's first roundabout was constructed around a megalithic passage tomb at Abbeyquarter North in Garavogue Villas. This is an outlier of the large group of monuments at Carrowmore on the Coolera Peninsula on the western outskirts of the town. The area around Sligo town has one of the highest densities of prehistoric archaeological sites in Ireland, and is the only place in which all classes of Irish megalithic monuments are to be found together. Knocknarea mountain, capped by the great cairn of Miosgan Maeve, dominates the skyline to the west of the town. Cairns Hill on the southern edge of the town also has two very large stone cairns.

Excavations for the National Roads Authority (NRA) for the N4 Sligo Inner Relief Road in 2002 revealed an early Neolithic causewayed enclosure. Built around 4000 B.C., the Magheraboy causewayed enclosure is located on high ground overlooking the town from the south. This is the oldest causewayed enclosure so far discovered in Britain or Ireland. It consists of a large area enclosed by a segmented ditch and palisade, and was perhaps an area of commerce and ritual. These monuments are associated with the coming of agriculture and hence the first farmers in Ireland. According to archaeologist Edward Danagher, who excavated the site, "Magheraboy indicates a stable and successful population during the final centuries of the fifth millennium and the first centuries of the fourth millennium BC". Danagher's work also documented a Bronze Age Henge at Tonafortes (beside the Carraroe roundabout) on the southern outskirts of Sligo town.

Sligo Bay is an ancient natural harbour, being known to Greek, Phoenician and Roman traders as the area is thought by some to be the location marked as the city of Nagnata on Claudius Ptolemy's second century A.D. co-ordinate map of the world. During the early medieval period, the site of Sligo was eclipsed by the importance of the great monastery founded by Columcille 5 miles to the north at Drumcliff. By the 12th century, there was a bridge and a small settlement in existence at the site of the present town.

The Norman knight Maurice Fitzgerald, the Justiciar of Ireland, is generally credited with the establishment of the medieval European-style town and port of Sligo, building Sligo Castle in 1245. The annalists refer to the town as a sraidbhaile ('street settlement') which seems to have consisted of the castle and an attached defensive bawn in the vicinity of Quay street. A Dominican Friary (Blackfriars) was also founded by Maurice Fitzgerald and the King of Connacht, Felim mac Cathal Crobderg Ua Conchobair, in 1253. This was accidentally destroyed by fire in 1414, and was subsequently rebuilt in its present form by Tighernan O’Rourke. Norman hegemony was, however, not destined to last long in Sligo. The Norman advance was halted in Sligo after the battle of Credran Cille in 1257 at Ros Ceite (Rosses Point) between Godfrey O'Donnell, Lord of Tirconnell, and Maurice Fitzgerald. Both commanders were mortally wounded in single combat. The Norman invasion of Tír Chonaill was abandoned after this. In 1289 a survey indicates there were 180 burgesses in the town. The Normans had laid a foundation that was to last.