The Third Division South of The Football League was a tier in the English association football league system from 1921 to 1958. It ran in parallel with the Third Division North with clubs elected to the League or relegated from Division Two allocated to one or the other according to geographical position. Some clubs in the English Midlands shuttled between the Third Division South and the Third Division North according to the composition of the two leagues in any one season.
In 1921 a Northern section was created, and the original division was renamed Third Division South. The original 22 teams in the Third Division formed the new Third Division South, with the exceptions of Crystal Palace, who were promoted to the Second Division, Grimsby Town who were transferred to the Third Division North, and Aberdare Athletic and Charlton Athletic who joined The Football League for the first time. Several Midlands-based teams were included in the Third Division South from time to time, although most were geographically closer to their Northern division rivals; Nottingham Forest and Notts County played in the Southern division although nearby Derby County spent time in the Northern division.
Only one promotion place was available each season from the Third Division South to the Second Division, which made it very difficult to win promotion. Six teams, Brighton & Hove Albion, Exeter City, Northampton Town, Southend United, Swindon Town, and Watford, were ever-present in the division for the 30 years of its existence. Of the teams that played in the Third Division South, Portsmouth, Ipswich Town, and Nottingham Forest were later English football champions.
Its final season was 1957–58, after which the North and South sections were merged to form a single Third Division and a new Fourth Division. The top 12 clubs in Division Three South, except for the Champions Brighton & Hove Albion, went into the new Third Division, and the bottom 12 clubs went into the Fourth Division.
From 1934 to the war's outbreak there was a short-lived knockout competition Football League Third Division South Cup.
From the 1954-55 season until the 1957-58 season, there was a series of games between teams representing the Third Division North and the Third Division South.
|1939–40||League abandoned due to World War II|
|1940–46||League suspended due to World War II|
|1947–48||Queens Park Rangers|
|1957–58||Brighton & Hove Albion|