Sukhothai, Trat and Rayong were relegated at the end of the 2020–21 season after finishing in the bottom three places of the table.

They were replaced by 2020-21 Thai League 2 champions Nongbua Pitchaya.

Note: Flags indicate national team as has been defined under FIFA eligibility rules.

The FIFA Transfer Window Period for Thailand was 4 May 2021 to 24 July 2021.

In order to preserve chronological evolvements, any postponed matches are not included in the round at which they were originally scheduled, but added to the full round they were played immediately afterwards.