Promoted to the 2018 Thai League 2 Promoted from the 2017 Thai League 4 Relegated to the 2018 Thai League 4 Northern Region Relegated to the 2018 Thai League 4 Western Region Relegated from the 2017 Thai League 2 This round was featured by Ayutthaya United, the second place of 2018 Thai League 3 Upper Region and Nara United, the second place of 2018 Thai League 3 Lower Region.

Winners of third place play-off would promoted to 2019 Thai League 2.

This round was featured by JL Chiangmai United, the first place of 2018 Thai League 3 Upper Region and MOF Customs United, the first place of 2018 Thai League 3 Lower Region.

Both winners and runners-up would promoted to 2019 Thai League 2 automatically.

JL Chiangmai United won 2–0 on aggregate.