The Thai League (Thai: ไทยลีก, pronounced as ไท-หลีก) or Thai League 1 (Thai: ไทยลีก 1, pronounced as ไท-หลีก-หนึ่ง), often referred to as T1, is the highest level of the Thai football league system. Contested by 16 clubs, it operates on a system of promotion and relegation with Thai League 2. Seasons typically run from August to May, with each team playing 30 games: two against each other team, one home and one away. It is sponsored by BYD Auto and therefore officially known as the BYD Sealion 6 League I. In the Thai League, most games are played on Saturday and Sunday evenings, occasionally being played on Wednesday and Friday evening fixtures.

Thailand has had organized football competition since 1916, when the Football Association of Thailand (then known as the Siam Football Association) established the first official tournament. The Kor Royal Cup (Thai: ถ้วย ก., "Cup Kor") served as the premier football competition from 1916 to 1995, operating as a knockout tournament rather than a league system. During this 79-year period, the tournament underwent several format changes, initially featuring only Bangkok-based clubs before expanding to include provincial teams in the 1960s.

The transition from cup to league format was driven by the need to modernize Thai football and meet Asian Football Confederation (AFC) standards for professional competitions. The semi-professional nature of the Kor Royal Cup, with its limited schedule and lack of home-and-away fixtures, was seen as inadequate for developing Thai football to international standards.

Thai League was introduced in 1996 by the Football Association of Thailand (FA Thailand) under the name Thailand Soccer League. Eighteen clubs who earlier competed for the Kor Royal Cup were registered to play in the first edition of a double round-robin league system. Bangkok Bank was crowned as the first champion of the 1996–97 Thailand Soccer League.

The Thai League originally had 10 to 12 clubs each season until 2007, when it was expanded to 16 clubs. At the end of each season, the three bottom placed clubs are relegated to the Thai Division 1 League.

Most of Thai League clubs in that time were the organisation of government authorities club that based in Greater Bangkok and Metropolitan. Meanwhile, the other local clubs had competed in the semi-pro league called the Provincial League. Thai Premier League faced the issue of low attendance and lack of local loyalties while the Provincial league suffered the financial issue. In 2007, Thai League was integrated with Provincial League completely. Chonburi from the Provincial League was the first champion of the new Thailand Premier League in the 2007 season.

In 2009 season, there were significant changes in the lead to the new era of the Thai Premier League. Asian Football Confederation declared the regulations for the associations that have the intention to send the clubs to compete in the AFC Champions League starting from 2011. Football Association of Thailand had to establish Thai Premier League Co., Ltd. and forced the clubs in the top league to complete AFC Club License Criteria otherwise Thai clubs will not eligible to play in the Champions League. Clubs were forced to separate themselves from the parent organisations and registered as the independent football authorities.

The massive changes occurred in that season. Thailand Premier League renamed to Thai Premier League. Two times league champion Krung Thai Bank failed to complete the new regulations. The organisation decided to sell the club. The club was acquired by Boon Rawd and rebranded to be Bangkok Glass. Bangkok University had expelled their football club section. The club rebrand itself to Bangkok United since then. The organisation-based clubs had to relocate to find the local supporters to backup the clubs. Osotspa changed their home stadium to Saraburi Province, TOT moved to play in Kanchanaburi, Royal Navy played in Rayong Province while Thailand Tobacco Monopoly integrated to Samut Sakhon Province and rebranded to TTM Samut Sakhon.