Southern Thailand is located on the Malay Peninsula, with an area around 70,713 km², bounded to the north by Kra Isthmus as the narrowest part of the peninsula. The western part has steeper coasts, while on the east side river plains dominate. The largest river of the south is the Tapi, which together with the Phum Duang drains more than 8,000 km², more than 10% of the total area of Southern Thailand. Smaller rivers include the Pattani, Saiburi, Krabi and the Trang. The biggest lake of the south is the Songkhla lake (1,040 km² altogether), the largest artificial lake is the Chiao Lan (Ratchaprapha dam) with 165 km² within the Khao Sok national park.

Running through the middle of the peninsula are several mountain chains, with the highest elevation at the 1835 m high Khao Luang in the Nakhon Si Thammarat Province. Ranging from the Kra Isthmus till the Phuket island is the Phuket chain, which connects to the Tanao Si Mountain Range further north. Almost parallel to the Phuket chain but 100km to the east is the Nakhon Si Thammarat or Banthat chain, which begins with the Samui island and ends at the Malaysian border at the Koh Ta Ru Tao archipelago. The border to Malaysia is formed by the Sankalakhiri range, sometimes subdivided into the Pattani, Taluban and Songkhla chain. At the border to Malaysia begins the Titiwangsa chain.