Thai, Central Thai,or Siamese, is the national and official language of Thailand and the first language of the Thai people and the vast majority of Thai Chinese. It is a member of the Tai group of the Tai–Kadai language family. Over half of its words are borrowed from Pali, Sanskrit, Mon, and Old Khmer. It is a tonal and analytic language.

Thai is the official language of Thailand, natively spoken by over 20 million people (2000). Standard Thai is based on Ayutthaya dialect, and register in the educated classes. In addition to Central Thai, Thailand is home to other related Tai languages. Although linguists usually classify these idioms as related, but distinct languages, native speakers often identify them as regional variants or dialects of the "same" Thai language, or as "different kinds of Thai". Thai includes the following dialects: Central Plains Thai, Capital Core Thai, Upper Central Thai, Southwestern Thai, and Khorat Thai.

Thai also has a complex orthography and system of relational markers. Spoken Thai is mutually intelligible with Laotian, the language of Laos; the two languages are written with slightly different scripts but are linguistically similar.

Other than compound words and words of foreign origin, most words are monosyllabic.

Chinese-language influence was strong until the 13th century when the use of Chinese characters was abandoned, and replaced by Sanskrit and Pali scripts. However, the vocabulary of Thai retains many words borrowed from Middle Chinese. Later most vocabulary was borrowed from Sanskrit and Pāli; Buddhist terminology terminology is particularly indebted to these. Indic words have a more formal register, and may be compared to Latin and French borrowings in English. Old Khmer has also contributed its share, especially in regard to royal court terminology. Since the beginning of the 20th century, however, the English language has had the greatest influence, especially for scientific, technical, international, and other modern terms. Many Teochew Chinese words are also used, some replacing existing Thai words (for example, the names of basic numbers; see also Sino-Xenic).

Thai has undergone various historical sound changes. Some of the most significant changes, at least in terms of consonants and tones, occurred between Old Thai spoken when the language was first written and Thai of present, reflected in the orthography.

Old Thai had a three-way tone distinction on live syllables (those not ending in a stop), with no possible distinction on dead syllables (those ending in a stop, i.e. either /p/, /t/, /k/ or the glottal stop which automatically closes syllables otherwise ending in a short vowel).

There was a two-way voiced vs. voiceless distinction among all fricative and sonorant consonants, and up to a four-way distinction among stops and affricates. The maximal four-way occurred in labials (/p pʰ b ʔb/) and dentals (/t tʰ d ʔd/); the three-way distinction among velars (/k kʰ ɡ/) and palatals (/tɕ tɕʰ dʐ/), with the glottalized member of each set apparently missing.

The major change between old and modern Thai was due to voicing distinction losses and the concomitant tone split. This may have happened between about 1300 and 1600 CE, possibly occurring at different times in different parts of the Thai-speaking area. All voiced–voiceless pairs of consonants lost the voicing distinction:

• Plain voiced stops (/b d ɡ dʐ/) became voiceless aspirated stops (/pʰ tʰ kʰ tɕʰ/).

• Voiced fricatives became voiceless.

• Voiceless sonorants became voiced.