County Louth, otherwise known as Louth County or Louth, is a former parliamentary constituency in Ireland, which was represented in the House of Commons of the Parliament of the United Kingdom. From 1801 to 1885 it returned two Members of Parliament (MPs), and one from 1918 to 1922.

From 1801 to 1885, the constituency comprised the whole of County Louth, except for the parliamentary boroughs of Drogheda and Dundalk. Between 1885 and 1918 the county was divided into the county division constituencies North Louth and South Louth. In 1918, the reunited constituency covered the entire county of Louth plus a small part of County Meath near Drogheda.

Louth was a constituency in the first Dáil election in December 1918 when Sinn Féin won by 255 votes, its narrowest margin of victory in that election. John J. O'Kelly, a native of Kerry, resident in Glasnevin (Dublin), was Louth's first TD. The constituency was merged with Meath to form the 5 seat Louth–Meath constituency for the 2nd and 3rd Dála. In 1923 Louth became a new 3 seat constituency.