Supplementary football leagues are association football competitions set up by clubs which are already members of more senior leagues to give the clubs a larger fixture list and fill in "spare" Saturdays which were not being used for cup-ties or league games.
A number of supplementary leagues were created in the early days of organised football in Scotland by Scottish Football League clubs as, with only 10 member clubs each playing just 18 games per season, there were plenty of spare Saturdays to fill. The gradual expansion of the Scottish League after 1900 eventually killed these off.