The 2021–22 Thai League 2 was the 24th season of the second-tier professional football league in Thailand since its inception in 1997. Officially known as the M-150 Championship due to sponsorship arrangements, the league was governed by the Football Association of Thailand (FAT). The season featured 18 clubs competing for promotion to the top-flight Thai League 1.
The competition followed a double round-robin format, with each team playing a total of 34 matches. The regular season commenced on 14 August 2021 and concluded on 30 April 2022. Following the conclusion of the regular season, a promotion play-off was held in May 2022 to determine the third club to be promoted.
At the end of the season, Lamphun Warriors finished at the top of the table to claim the championship title, marking their first title in the second tier and securing direct promotion to the 2022–23 Thai League 1. Sukhothai finished as runners-up, also earning direct promotion. The third promotion spot was contested in the play-offs between the teams finishing third through sixth (Lampang, Trat, Phrae United, and Chainat Hornbill). Lampang emerged as the play-off winner to secure their advancement to the first tier.
In terms of relegation, the bottom three teams were originally slated for demotion to the Thai League 3. Khon Kaen and Navy finished in the 17th and 18th positions, respectively, and were relegated. Although Customs Ladkrabang United finished in 16th place (a relegation position), they maintained their status in the Thai League 2 for the following season. This occurred because Muangkan United, who finished 7th, failed to obtain a club license for the 2022–23 season, resulting in their administrative relegation and the reprieve of the 16th-placed team.
The 2021–22 season was impacted by the ongoing COVID-19 pandemic, requiring clubs to adhere to specific health protocols and government regulations regarding spectator attendance and stadium safety throughout the campaign.
