Koh Lanta Yai is in the Andaman Sea off the west coast of Thailand, between the Phi Phi Islands and the mainland. It is administratively part of Krabi Province, most of which is on the mainland. Together with neighboring Koh Lanta Noi which forms Koh Lanta archipelago and several other islands it forms the amphoe of Ko Lanta District.
The island is 25 km (16 mi) long and 6 km (3.7 mi) wide, with an area of 81 km2 (31 sq mi), and a small portion of it belongs to the Mu Ko Lanta National Park. The area was designated as the 62nd national park of Thailand in 1990.