Thailand changed its official name from Siam to Thailand in 1939.
The name Thailand comes from the Thai word thai, commonly understood as “free,” although the term also refers to the Thai people. The 1939 change took place during the government of Field Marshal Plaek Phibunsongkhram and formed part of a broader campaign of national cultural reforms.
The country briefly returned to the name Siam after the Second World War, but the official name Thailand was restored in 1949. In English, “Siam” remains familiar because it was used in historical accounts, the former Siamese monarchy and expressions such as Siamese twins.
Thailand is a constitutional monarchy in mainland Southeast Asia. Its capital, Bangkok, was known in formal historical usage by a much longer ceremonial name, while “Thailand” is the country’s standard modern English name.