Tun Sun (Chinese: 頓遜) or Tian-Sun or Tien-Sun (Chinese: 典遜; Thai: เทียนสน) or Tu-k'un/Tou-k'ouen/Ch'u-tu-k'un (都昆), was a group of five ancient Mon political entities, stretching from present-day lower central Thailand to the Kra Isthmus in southern Thailand and Tanintharyi Region of Myanmar. It existed from the 1st to 6th centuries CE, and was said to have stretched from the east to the west coast, controlling a vital branch of long-distance maritime trade between the India Ocean and the South China Sea. It was also one of the earliest Indianized-states in Southeast Asia.

In the late 1st century, Tun Sun once vassalized its southern neighbor, Gē Luó Fù Shā (哥羅富沙), later known as Kamalanka), on the Kra Isthmus and Bandon Bay area, as mentioned in the Guangdong Tongzhi [zh]. Tun Sun remained independent until it was seized and became a vassal of Funan in the 3rd century CE, at least before 245 CE. Tun Sun disappeared from history about the beginning of the 6th century CE, when new principalities of Dvaravati emerged in central Thailand. Kamalanka or Lang-ya-hsiu, centered at the ancient Nakhon Pathom, was expected to be the successor of Tun Sun.

Roland Braddell proposed that an ancient emporium mentioned in the Geōgraphikē Hyphēgēsis of Ptolemy and the Buddhist texts, Mahāniddesa and Milinda Panha, in the 2nd–4th centuries, was one of the principalities in Tun Sun. The others four kingdoms of Tun Sun might be the vassalized Gē Luó Fù Shā, as cited in the Guangdong Tongzhi, and the three brothers of Gē Luó Shě Fēn (哥罗舍分国; Kamalanka), Xiū Luó Fēn, and Gān Bì (甘毕), as it was said to be situated in western Menam Valleys to the west of Dvaravati, where the three significant historical towns of the ancient Nakhon Pathom, Khu Bua, and Mueang Uthong are located.

Since Tun Sun was partitioned among five monarchs, the etymology of the name given by the Chinese is highly relevant: the number "5" in the ancient Mon Nyah Kur language, spoken by thousands of ancient Mon Dvaravati Nyah Kur people, is pronounced Chuun, while Duinr means "kingdom, country". This has been interpreted to mean that Tun Sun was a confederation of five territories. It also incidentally corresponded with the legendary state of Suvarnabhumi, claimed by Thai scholars to exist from the late 2nd century BCE to 2nd century CE in modern central Thailand, which consists of five king cities in the upper Malaysian peninsula, including Suvarnabhumi (the present-day old town of Nakhon Pathom), Ratchaburi, Mueang Sing, Phetchaburi, and Tanintharyi, probably with another crown dependency, Lamphakappa Nakhon (ลัมภกัปปะนคร, present-day Lampang) in the north.

Even the exact location of Tun Sun remains unclear. Still, scholars suggest that it was likely on the upper Malaysian peninsula, possibly near Pong Tuk (พงตึก) in modern Kanchanaburi province or Phra Pathom area in the old town Nakhon Pathom in central Thailand, which was dominated by the Mon people in the respective period. The Chinese reported that Tun Sun is located more than 3,000 li from the southern frontier of Funan. Its territory covered 1,000 li in width stretching from the east to the west coast with the political center situated 50 li (25 kilometers) from the sea; thus, it should be Pong Tuk instead of the old town Nakhon Pathom, which was located near the ancient Dvaravati coast. Its border with the northern neighboring entity, Chin-lin, was ill-defined, but probably not far above the present-Mergui-Tanintharyi Region, while the south encountered the kingdom of Chü-li (Takola; speculated to be Pan Pan) near the Kra Isthmus. It bordered with Jiaozhou (交州) to the east.

Other scholars, a Singaporean Han Wai Toon and H.G.Q. Wales, on the other hand, suggests that Tun Sun might probably situated at the southern tip of the Malay Peninsula near the village of Johor Lama in Johor, Malaysia, but the presupposition was rebutted by Paul Wheatley due to lacking evidence to support the ancient flourishment.

Roland Braddell proposed that Tun Sun was rather less than one-quarter of the distance from Chü-li, which Braddell believed to be Kou-chih of Kole polis in present-day near Kuantan of Malaysia to Funan according to the text given in Lo yang chia lan chi about the Bodhibhadra's voyage states that Tun Sun was some thirty days' sail from Funan and eleven days northwards from Kou-chih. This corresponded with a Thai scholar Ruangyot Jantrakiri (เรืองยศ จันทรคีรี) who said Tun Sun was founded in 52 CE and was situated in the present-day Na Tham sub-district (ตำบลหน้าถ้ำ), Yala province, in deep south Thailand. However, several historians confirmed that the area mentioned by Jantrakiri was probably Chi Tu.

The political structure of Tun Sun is difficult to ascertain. It hosted a colony of South Asians including over 1,000 Brahmans. It was mentioned in the 3rd century CE in the Nan-chou I-wu Chinh, a Chinese in which it is noted that the people of Tun Sun practiced intermarriage with Brahmans from India who were, reportedly, very pious. Five hundred families of hu (胡), who were believed to be Mongol and Tartar tribes from Central Asia as well as two (hundred?) fo-t'u were also present in the country.