Southern Thai (ภาษาไทยถิ่นใต้ [pʰaːsǎː tʰaj tʰìn tâːj]), also known as Dambro (ภาษาตามโพร [pʰaːsǎː taːm pʰroː]), Pak Tai (ภาษาปักษ์ใต้ [pʰaːsǎː pàk tâːj]), or "Southern language" (ภาษาใต้ [pʰaːsǎː tâːj]),^{[citation needed]} is a Southwestern Tai ethnolinguistic identity^[2] and language spoken in southern Thailand as well as by small communities in the northernmost Malaysian states. It is spoken by roughly five million people, and as a second language by the 1.5 million speakers of Pattani and other ethnic groups such as the local Peranakans communities, Negritos, and other tribal groups.^{[citation needed]} Most speakers are also fluent in or understand the Central Thai dialects.

Southern Thai is classified as one of the Chiang Saen languages—others being Thai, Northern Thai and numerous smaller languages, which together with the Northwestern Tai and Lao-Phutai languages, form the Southwestern branch of Tai languages. The Tai languages are a branch of the Kra–Dai language family, which encompasses a large number of indigenous languages spoken in an arc from Hainan and Guangxi south through Laos and Northern Vietnam to the Cambodian border.

Phonyarit (2018)^[3] recognizes the following nine main dialects of Southern Thai, based on tone split and merger patterns.

In Thailand, speakers of Southern Thai can be found in a contiguous region beginning as far north as southern part of Prachuap Khiri Khan Province and extending southward to the border with Malaysia. Smaller numbers of speakers reside in the Malaysian border states, especially Kedah, Kelantan, Penang, Perlis, and Perak. In these areas, it is the primary language of ethnic Thais as well as of the ethnically Malay people on both sides of the Thai-Malaysian border in Satun and Songkhla provinces. Although numerous regional variations exist and there is no one standard, the language is most distinct near the Malaysian border. All varieties, however, remain mutually intelligible. For economic reasons, many speakers of Southern Thai have migrated to Bangkok and other Thai cities. Some have also emigrated to Malaysia, which offers not only economic opportunity but also a culture which shares the Islamic faith practiced by some speakers of Southern Thai.

Malay kingdoms ruled much of the Malay Peninsula,^{[citation needed]} such as the Pattani Kingdom and Tambralinga, but most of the area, at one time or another, was under the rule of Srivijaya. The population of the Malay peninsula was heavily influenced by the culture of India transmitted through missionaries or indirectly through traders. Numerous Buddhist and Hindu shrines attest to the diffusion of Indian culture. The power vacuum left by the collapse of Srivijaya was filled by the growth of the kingdom of Nakhon Si Thammarat, which subsequently became a vassal of the Sukhothai Kingdom. The area has been a frontier between the northern Tai peoples and the southern ethnic Malays as well as between Buddhism and Islam.

Although of the major regional languages of Thailand, Southern Thai is most similar in lexicon and grammar to Central Thai, the varieties are sufficiently different that mutual intelligibility between the two can be problematic. Southern Thai presents a diglossic situation wherein registers range from the most formal (Standard Central Thai spoken with Southern Thai tones and accent) to the common vernacular (usually a contracted form of Thai expressions and with some amount of loan words from Malay). The Thai language was introduced with Siamese incursions into the Malay Peninsula possibly starting as early as the Sukhothai Kingdom. During this and successive kingdoms, the area in which Southern Thai is spoken was a frontier zone between Thai polities and the Malay Sultanates.^{[citation needed]} Malay vocabulary has been absorbed into the lexicon, as a considerable number of Malay speakers lived in or near Patani polity and interacted with the Thai speakers through trade; and the Malay language was formerly considered to be a lingua franca of the southern part of the Malay peninsula.

Southern Thai is mainly a spoken language, although the Thai alphabet is often used in the informal situations when it is written.

The words used that are etymologically Thai are often spoken in a reduced and rapid manner, making comprehension by speakers of other varieties difficult. Also, as Southern Thai uses up to seven tones in certain provinces, the tonal distribution is different from other regional varieties of Thai. Additionally, Southern Thai speakers almost always preserve ร as /r/ in contrast to Northern Thai, the Lao-based Isan language, and informal registers of Central Thai where it is generally realized as /l/.