County Louth ( /ˈlaʊð/; Irish: Contae Lú) is a county of Ireland. It is part of the Border Region and is also located in the province of Leinster. It is named after the town of Louth. Louth County Council is the local authority for the county. The population of the county is 122,808 according to the 2011 census.

County Louth is colloquially known as 'the Wee County' as it is the smallest county by area (826 km² (319sq miles). It is the 19th largest in terms of population. It is the smallest of Leinster’s 12 counties by size and the sixth largest by population.

The official spelling in the Irish (Gaelic) language (Lú) must not be confused with the similar sounding superlative form lú meaning smaller or smallest of the adjective beag. Instead, it is derived from Lughbhaidh - the Celtic pagan god Lugh, whose festival was celebrated at Lughnasadh (Lúnasa). This is a county steeped in myth, legend and history, going back to the pre-historical days of the Táin Bó Cúailnge (Cooley Cattle Raid, see Cú Chulainn). Later it saw the influence of the Vikings as seen in the name of Carlingford Lough. There are a number of historic sites in the county, including religious sites at Monasterboice and Mellifont Abbey. In the early fourteenth century the Scottish army of Edward Bruce (brother of Robert of Bannockburn fame) was defeated in the Battle of Faughart near Dundalk, Edward losing not only his claim to the High Kingship Of Ireland, but also his life. The sixteenth and seventeenth centuries featured many skirmishes and battles involving Irish and English forces. Oliver Cromwell attacked Drogheda in 1649 slaughtering the Royalist garrison and hundreds of the town's citizens (Siege of Drogheda). Towards the end of the same century the armies of the warring Kings, James and William, faced off in South Louth during the build-up to the Battle of the Boyne - the battle takes its name from the river Boyne which reaches the sea at Drogheda.