Mayo South was a parliamentary constituency represented in Dáil Éireann, the lower house of the Irish parliament or Oireachtas from 1923 to 1969.

The method of election was proportional representation by means of the single transferable vote (PR-STV).

[1] From 1923 to 1948, Mayo South elected 5 deputies (Teachtaí Dála, commonly known as TDs).

Following the resignation of Fianna Fáil TD Micheál Clery, a by-election was held on 4 December 1945.

The seat was won by the Clann na Talmhan candidate Bernard Commons