Ranong is located on the Kra Isthmus, a narrow strip of land that connects Thailand with the Malay Peninsula, on the west of the Phuket mountain range. It has a long coast on the Andaman Sea. The province is known for having the highest rainfall in Thailand, the rainy season there lasting for about eight months. Its annual rainfall in 1955 totaled 6,699.5 mm, compared to about 1,200 in central Thailand.
Ranong is the least populated province; 80% of its area is covered by forests, and 67% is mountainous. In former years the major industry was tin mining, but most mines are now exhausted. White clay mining (for the production of porcelain) and fishing are now the main industries, along with rubber and cashew nuts.
The Ranong Biosphere Reserve in the north of Amphoe Kapoe, covering 303.09 km², was declared in 1997. It is the fourth biosphere reserve of Thailand, and the only one at the seacoast to protect the mangrove forests. Khlong Nakha Wildlife Sanctuary is in the south of the province.