Mayo South–Roscommon South was a parliamentary constituency represented in Dáil Éireann, the lower house of the Irish parliament or Oireachtas from 1921 to 1923. The constituency elected 4 deputies (Teachtaí Dála, commonly known as TDs) to the Dáil, on the system of proportional representation by means of the single transferable vote (PR-STV).

The constituency was created in 1921, under the Government of Ireland Act 1920, for the 1921 general election to the House of Commons of Southern Ireland, whose members formed the 2nd Dáil. It was used again for the 1922 general election to the 3rd Dáil. It covered part of the territory of the two counties of Mayo and Roscommon.

It was abolished under the Electoral Act 1923, and replaced by the two new constituencies of Mayo South and Roscommon.

Note: The columns in this table are used only for presentational purposes, and no significance should be attached to the order of columns. For details of the order in which seats were won at each election, see the detailed results of that election.

• Dáil constituencies
• Politics of the Republic of Ireland
• Historic Dáil constituencies
• Elections in the Republic of Ireland