The 2017–18 season was the 115th in the history of the Southern League since its establishment in 1894.
[1] At the end of the season a new division was to be created at step 3.
Also, the number of clubs in step 3 divisions was reduced from 24 to 22.
Hereford, Mangotsfield United, and North Leigh declined to participate.
The defending champions, Hayes & Yeading United, were defeated in the semi-finals by Hitchin Town.