The Football League Third Division South was a level of English professional football which ran in parallel to Third Division North from 1921 to 1958.

The division was created in 1920 as the Third Division, when the Football League absorbed the entire top division of the Southern League, effectively ending the ambitions of that league to rival the Football League. At that time, most of the Football League's strongest clubs were from the industrial areas of the North and Midlands, with only 7 of its 44 members based in the South (Arsenal, Chelsea, Clapton Orient, Fulham, Tottenham Hotspur and West Ham United, all from London, and Bristol City). The following season, a Northern section was created, and the original division was renamed Third Division South.

The division originally had 22 teams, expanding to 24 in 1950. Only one promotion place was available to the Second Division, meaning that many clubs spent long periods of time stuck in the division, and several were never promoted in the division's 31 seasons. At the end of each season, the bottom two clubs were put up for re-election, alongside the bottom two in the Northern section, although most survived the re-election vote.

In 1958, the North and South sections were merged together, to form a single Third Division and a new Fourth Division.