Ko Lanta Yai is an island in the Strait of Malacca 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 Ko Lanta Noi, which forms Ko Lanta archipelago and several other islands it forms the amphoe of Ko Lanta District.
The island is 25 km long and 6 km wide, with an area of 81 km², 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.
Recently, it has become well known, especially in Sweden. The island is relatively less developed with most of the accommodation available being basic bamboo huts. In 2015, a new bridge was built connecting it to the island of Ko Lanta Noi. There are very few paved roads on the island, mostly in the north, and they deteriorate gradually towards the south.
Ban Saladan, the main town and port, is at the northern tip of the island and is served by ferries from Krabi, Phuket, and Phi Phi. The main beaches are on the western shores of the island, the largest being Klong Dao, Pra-Ae, and Klong Khong Beach.