Croydon South was a constituency represented in the House of Commons of the Parliament of the United Kingdom. It elected one Member of Parliament (MP) by the first past the post system.

This seat should not be confused with the current Croydon South constituency

It was created for the 1918 general election when the County Borough of Croydon had grown so the Croydon seat was split into two seats.

In 1974 the seat was redrawn and renamed Croydon Central; following the 1965 addition of Purley and Coulsdon to the London Borough of Croydon in 1974 a fresh seat of Croydon South was created to the south predominantly from East Surrey rather than from the historic Croydon South.

It did not exist from 1950 to 1955 as an east–west division of the town was chosen instead. Its voters elected twice Labour candidates, for the 1945 and 1966 Parliaments when the Labour Party received strong majorities, and at all other elections elected Conservative candidates.

1918-1950: The County Borough of Croydon wards of Central, East, South, and West.

1955-1965: The County Borough of Croydon wards of Addington, Broad Green, Central, Shirley, South, and Waddon.

1965-1974: The London Borough of Croydon wards of Addington, Broad Green, Central, Shirley, South, and Waddon.

The seat was created in 1918 and the first MP was Ian Malcolm, who had been the MP for all of Croydon. H.T. Muggeridge, father of Malcolm Muggeridge, fought the seat for Labour four times from 1918, later becoming MP for Romford. The seat saw a by-election in 1932, won by Herbert Williams.

Croydon South had twice seen Croydon's only Labour MPs before the 1990s. David Rees-Williams held the seat from the 1945 Labour landslide until unfavourable boundary changes in 1950. David Winnick won the seat in 1966 before losing in 1970. Otherwise the seat, and indeed the rest of Croydon, had always been firm Conservative territory.

From 1950 until 1955 the seat was divided into east and west, represented by Conservatives Herbert Williams and Richard Thompson respectively.