The 2005–06 season was the 91st in the history of the Isthmian League, which is an English football competition featuring semi-professional and amateur clubs from London, East and South East England. It was the last season in which the Isthmian League consisted of three tiers. At the end of the season Division Two was disbanded, and most of the Division Two clubs were distributed between lower level leagues, while Division One was divided into Division One North and Division One South.