County Carlow was a parliamentary constituency in Ireland, which from 1801 to 1885 returned two Members of Parliament (MPs) to the House of Commons of the United Kingdom, and one MP from 1885 to 1922.

County Carlow had been represented by two seats in the Irish House of Commons. Under the Acts of Union 1800, it continued to be represented by two MPs, now in the United Kingdom House of Commons. It comprised the whole of County Carlow, except for the borough of Carlow, which was separately represented from 1801 to 1885. The borough of Old Leighlin was disfranchised under the Acts of Union 1800.

Under the Redistribution of Seats Act 1885, the borough of Carlow was disfranchised and the county was reduced to one seat.^[1] It was the only Irish county not divided for electoral purposes in the 1885 redistribution. It was thus the only Irish county constituency to exist at every general election from the union with Great Britain to the establishment of the Irish Free State.

It was not affected by the Redistribution of Seats (Ireland) Act 1918. The 1918 general election was used by Sinn Féin as the first election to Dáil Éireann. James Lennon sat as a member of the First Dáil, abstaining from Westminster.

Under the Government of Ireland Act 1920, it was combined with the constituencies of North Kilkenny and South Kilkenny to form Carlow–Kilkenny as a 4-seat constituency for the Southern Ireland House of Commons and a one-seat constituency at Westminster.^[2] At the 1921 election for the Southern Ireland House of Commons, the four seats were won uncontested by Sinn Féin, who treated it as part of the election to the Second Dáil. James Lennon was one of the four TDs elected for Carlow–Kilkenny. It was never used as a Westminster constituency; under the Irish Free State (Agreement) Act 1922, no writ was to be issued "for a constituency in Ireland other than a constituency in Northern Ireland".^[3] Therefore, following a dissolution on 26 October 1922, no vote was held in Carlow–Kilkenny at the 1922 United Kingdom general election on 15 November 1922. The Irish Free State left the United Kingdom on 6 December 1922.

Notable MPs for County Carlow included Nicholas Aylward Vigors, a zoologist, John Ball, a naturalist and Under-Secretary of State for the Colonies, Arthur MacMurrough Kavanagh, and James Patrick Mahon.

• List of United Kingdom Parliament constituencies in Ireland and Northern Ireland
• Historic Dáil constituencies