29 clubs will be divided into 2 groups (regions).

Winners of third place play-off would promoted to 2018 Thai League 2.

Udon Thani won on away goals.

This round was featured by Khonkaen, the first place of 2017 Thai League 3 Upper Region and Samut Sakhon, the first place of 2017 Thai League 3 Lower Region.

Both winners and runners-up would promoted to 2018 Thai League 2 automatically.