Siam

Up until 1939 Thailand was known as Siam.
The  Siam national football team played its first match on December 20th 1915. The opponents were a team of mixed European residents. Rather imaginatively this team was called  The Europeans. The match was held at Bangkok's Royal Sport Club. I do not know what the result was. 
On April 25, 1916, King Rama VI (Vajiravudh) established the Football Association of Siam and instituted The Yai Cup.  King Rama VI appears to have been an interested and active patron of the game. 
Siam joined FIFA in 1925, the first Asian country to do so.
The team played its first international match in 1930 against an Indochina team made up of Vietnamese and French players.

1915 : Siam team with the King

The  players who faced The Europeans in 1915

1916: Department of Performing Arts pictured with the Yai Cup (donated by The King).

 Yai Cup Winners 

1916	Department of Performing Arts
1917	Vajiravudh College
1918	Vajiravudh College
1919	Vajiravudh College
1920	Chulalongkorn University
1921	Royal Military Academy
1922	Royal Military Academy
1923	Royal Thai Naval Academy
1924	Royal Thai Naval Academy
1925	No competition
1926	Kong Dem Rot
1927	Kong Dem Rot
1928	Suankularb Wittayalai School
1929	Suankularb Wittayalai School
1930	Assumption Academy
1931	Thailand Post