Thai, or Central Thai (historically Siamese), is a Tai language of the Kra–Dai language family spoken by the Central Thai people and a vast majority of Thai Chinese. It is the sole official language of Thailand.

Thai is the most spoken of over 60 languages of Thailand by both number of native and overall speakers. Over half of its vocabulary is derived from or borrowed from Pali, Sanskrit, Mon and Old Khmer. It is a tonal and analytic language. Thai has a complex orthography and system of relational markers. Spoken Thai, depending on standard sociolinguistic factors such as age, gender, class, spatial proximity, and the urban/rural divide, is partly mutually intelligible with Lao, Isan, and some fellow Thai topolects. These languages are written with slightly different scripts, but are linguistically similar and effectively form a dialect continuum.

Thai language is spoken by over 69 million people (2020). Moreover, most Thais in the northern and the northeastern parts of the country today are bilingual speakers of Central Thai and their respective regional dialects because (Central) Thai is the language of television, education, news reporting, and all forms of media. A recent research found that the speakers of the Northern Thai language (or Kham Mueang) have become so few, as most people in northern Thailand now invariably speak Standard Thai, so that they are now using mostly Central Thai words and seasoning their speech only with “kham mueang” accent. Standard Thai is based on the register of the educated classes by Central Thai people in the Metropolis.

As a dominant language in all aspects of society in Thailand, Thai initially saw gradual and later widespread adoption as a second language among the country’s minority ethnic groups from the mid-late Ayutthaya period onward. Ethnic minorities today are predominantly bilingual, speaking Thai alongside their native language or dialect.