Ko Samui or Koh Samui (Thai: เกาะสมุย) is an island off the east coast of the Kra Isthmus in Thailand, close to the mainland Surat Thani town. Samui is Thailand's third largest island, with an area of 228.7 km².

The island was probably first inhabited about 15 centuries ago, settled by fishermen from Malay Peninsula and Southern China. It appears on Ming Dynasty maps dating back to 1687, under the name Pulo Cornam. The name Samui is mysterious in itself. Perhaps it is an extension of the name of one of the native trees, mui, or it is a corruption of the Chinese word Saboey, meaning.

Today, Samui has a population of nearly fifty thousand, and lives on a successful tourist industry, as well as exports of coconut and rubber. It even has its own international airport, Samui Airport, with flights daily to Bangkok, Hong Kong, Singapore and other major airports in Asia.

Samui is located in the Gulf of Thailand, about 35km northeast of Surat Thani town (9°N, 100°E). It is surrounded by about sixty other islands, most of which comprise the Ang Thong National Marine Park, but also include other tourist destinations Ko Pha Ngan, Ko Tao and Ko Nang Yuan.

The old capital is Nathon, on the southwest coast of the island, and remains the major port for fishing and inter-island transportation. Nathon is the seat of the regional government, and the true commercial hub of the Samui locals. It has a charming pace, and is almost small enough to walk everywhere. The old Chinese shop houses along the middle street wisper of an exotic history.

Each of Samui's primary beaches is now also nominally considered a town, due to the number of hotels, restaurants and bars that have sprung up in recent years.