Japanese Association in Thailand (タイ国日本人会、website only in Japanese) was established in 1912 as the Japanese Club (日本人倶楽部). That makes it world’s oldest organisation of that kind, well ahead of any other country with Japanese presence. One of its accomplishments was acquiring a plot of land on the supposed site of the 17th-century Japanese settlement in Ayutthaya and establishing there first a memorial site and later an open-air museum.
The Thai-Japanese Association (泰日協会, ), estasblished in 1935, is another kettle of fish. With its website in English and Thai, but no Japanese, it aims, quite successfully, at promoting various sorts of cooperation between the two countries.