The Royal Thai General System of Transcription (RTGS) is the official system for rendering Thai words in the Latin alphabet. It was published by the Royal Institute of Thailand in early 1917, when Thailand was called Siam.

It is used in road signs and government publications and is the closest method to a standard of transcription for Thai, but its use, even by the government, is inconsistent. The system is almost identical to the one that is defined by ISO 11940-2.

Prominent features of the system are:

Final consonants are transcribed according to pronunciation, not Thai orthography.

Vowels are transcribed in the position in the word where they are pronounced, not as in Thai orthography. Implied vowels, which are not written in Thai orthography, are transcribed as pronounced.

A hyphen is used to avoid ambiguity in syllable separation before a succeeding syllable that starts with a vowel and before ⟨ng⟩ if the preceding syllable ends with a vowel.

Transcribed words are written with spaces between them although there are no spaces in Thai. For example, สถาบันไทยคดีศึกษา Institute of Thai Studies is transcribed as Sathaban Thai Khadi Sueksa. However, compounds and names of persons are written without spaces between words. For example, ลูกเสือ (from ลูก + เสือ, 'scout') is transcribed as luksuea, not luk suea, and โชคชัย จิตงาม, the first and last names of a person, is transcribed as Chokchai Chitngam, not Chok Chai Chit Ngam.

For consonants, the transcriptions are given for both initial and final position in the syllable. For vowels, a dash ("–") indicates the relative position of the vowel's initial consonant.