The Isthmian League is a football league from the UK. Also known as the Bostik league due to its sponsorship, the league covers London as well as East and South East England and comprises primarily of semi-professional football clubs.

Formed in London in 1905 by local amateur football clubs, the Isthmian Football League now comprises of 72 teams and has 3 divisions (the North and South feeder divisions and the Premier League). The Northern Premier League, Southern League, and Isthmian League form the 7th and 8th English Football League level, is fed from a range of regional leagues, and feeds primarily to the National League (South).

The Isthmian Football League took a leap from being an amateur-only league to accepting professionals in the 70s. The 2nd division was formed in 1973 when the league had 16 clubs, and this was split between its North and South divisions in 1984. The 3rd division was formed in 1977.

After further restructuring in 1991, these were reassigned to the 2nd and 3rd divisions. There were more restructuring in 2002, which saw the amalgamation of the first 2 divisions forming the North and South regions of the first division (the 3rd division became the second division).

2004 was also about the time that the Football Association went through a lot of restructuring of its own, particularly where the non-league National League system was concerned. This saw the creation of new regional Football Conference divisions, and as a result, the Isthmian Football League was pruned down to 3 divisions as well as a boundary amendment which resulted in a Southern Football League overlap. Only 2 years later, the FA reverted back to 2 1st regional division with 2nd divisions being disbanded altogether. Since May 2017, the Isthmian League has been selected by the FA to add a 4th step division in a bid to restructure the National League system in reducing all 4th step divisions to only 20 teams.