Thai Sign Language (TSL) is primarily used by deaf Thai people. It is a language as unique to Thailand as spoken Thai is. It is said to have been around as old as spoken Thai.
Around the time The Thai Sign Language Dictionary, Book One was published in 1986, it states "this work will encourage the Royal Thai Government to formally recognize the Thai Sign Language as a legitimate Thai language." [p2] The Thai Minister of Education signed a resolution on behalf of the Royal Thai Government in August 1999, recognizing Thai Sign Language as "the national language of deaf people in Thailand."
Suwanarat, Manfa and Reilly, Charles. The Thai Sign Language Dictionary. Association of the Deaf in Thailand, 1986. Printed by Thai Watana Panich Press Col, Ltd.
"Thai Sign Language." http://www.thailanguage.org/thai/signlanguage.asp