Southern Thailand is on the Malay Peninsula, with an area of around 70,714 km2 (27,303 sq mi), bounded to the north by Kra Isthmus, the narrowest part of the peninsula. The western part has highly steep coasts, while on the east side river plains dominate. The largest river of the south is the Tapi in Surat Thani, which together with the Phum Duang in Surat Thani drains more than 8,000 km2 (3,100 sq mi), more than 10 percent of the total area of southern Thailand. Smaller rivers include the Pattani, Saiburi, Krabi, and the Trang. The biggest lake of the south is Songkhla Lake (1,040 km2 (400 sq mi) altogether). The largest artificial lake is the Chiao Lan (Ratchaprapha Dam), occupying 165 km2 (64 sq mi) of Khao Sok National Park in Surat Thani. The total forest area is 17,964 km2 (6,936 sq mi) or 24.3 percent of provincial area.

Ko Lao Liang Phi Running through the middle of the peninsula are several mountain chains, with the highest elevation at Khao Luang, 1,835 m (6,020 ft), in Nakhon Si Thammarat Province. Ranging from the Kra Isthmus to Phuket Island is the Phuket chain, which connects to the Tanao Si Mountain Range further north. Almost parallel to the Phuket chain, but 100 km (60 mi) to the east is the Nakhon Si Thammarat or Banthat chain, which begins with Samui Island, Ko Pha Ngan, and Ko Tao in Surat Thani Province and ends at the Malaysian border at the Ko Ta Ru Tao archipelago. The border with Malaysia is formed by the Sankalakhiri range, sometimes sub-divided into the Pattani, Taluban, and Songkhla chain. At the Malaysian border the Titiwangsa chain rises up.

The limestone of the west coast has been eroded into many steep singular hills. The parts submerged by the rising sea after the last ice age now form many islands, like the well-known Phi Phi Islands.[citation needed] Also well known is the so-called James Bond Island in Phang Nga Bay, featured in the movie The Man with the Golden Gun.

The population of the growing region is projected to be 9,156,000 in 2015, up from 8,871,003 in 2010 (census count and adjusted), despite these figures are adjusted for citizens who have left for Bangkok or who have moved to the region from elsewhere, as well as registered permanent residents (residency was problematic in the prior 2000 census), the figure is still misleading.[citation needed] There are still a huge number of migrant or informal workers, temporary workers, and even stateless people, and a large expatriate population which is not included.

Most of Southern Thailand is in Tenasserim-South Thailand semi-evergreen rain forests ecoregion. The Peninsular Malaysian rain forests and Peninsular Malaysian montane rain forests ecoregions extend into southernmost Thailand along the border with Malaysia.