The Thai Premier League 2009 was the thirteenth season of the Premier League since its establishment in 1996. A total of 16 teams competed in the league, with PEA FC as the defending champions.

Muang Thong United took the league championship with two names remaining, the first ever team to win the championship in their debut appearance in the top flight.

Nakhon Pathom, Sriracha and Chula United were relegated to the 1st Division.