Khao Sok National Park in Phanom district, Surat Thani province, covers an area 739 km² and includes the 165 square kilometre Cheow Lan Lake created by the Ratchaprapha Dam. The park is the largest area of virgin forest in southern Thailand and is a remnant of a much older and more diverse rain forest than the Amazon.

Beautiful sandstone and mudstone rocks rise about 300–600 m above sea level. The park is traversed by a limestone mountain range from north to south with a high point of 950 m. This mountain range is hit by monsoon rain coming from both the Gulf of Thailand and the Andaman Sea, which makes it one of Thailand's wettest regions. Heavy rainfall and falling leaves have led to the erosion of the limestone rocks and created the significant karst formations seen today.

The park is estimated to contain over five percent of the world's species. Wild mammals include Malayan tapir, Asian elephant, tiger, sambar deer, bear, gaur, banteng, serow, wild boar, pig-tailed macaque, langur, white handed gibbons, squirrel, muntjak, mouse deer and barking deer. The world's only known amphibious centipede, Scolopendra cataracta, was discovered on a stream bank near the national park in 2001.
Bamboo binds the topsoil, preventing soil erosion on hillsides and riverbanks. With more than 1,500 species, bamboo is the oldest grass in the world, dating back nearly 60 million years.

The park was established on 22 December 1980