Ko Pha-ngan is an island located in the Gulf of Thailand within Surat Thani Province of southern Thailand. It sits halfway between the larger island of Ko Samui to the south and the smaller Ko Tao to the north, forming part of the Chumphon Archipelago. Administratively, it constitutes the district of Amphoe Ko Pha-ngan alongside Ko Tao and several minor islands. The main town and primary ferry port is Thong Sala, which serves as the island's capital and central hub for transportation and commerce.

The name Ko Pha-ngan derives from the southern Thai word for sandbar, reflecting the many offshore sandbars in the area. Evidence of settlements dating back over 2,000 years includes a Bronze Drum of Dongson Culture found on nearby Ko Samui. Historically favored by past kings, King Chulalongkorn visited the island fourteen times during his reign. The minor district was established on October 1, 1970, and upgraded to a full district on April 12, 1977, with Ko Tao subdistrict splitting off in 1982.

The island is characterized by its coconut trees and diverse geography, including volcanic peninsulas and white sandy beaches. Notable sites include Haad Yao, Haad Salad, and the isolated Bottle Beach, accessible by boat or hike. Chalok Lam is a fishing village at the northern tip, while Thong Nai Pan offers twin bays for families. The island also features the Sanctuary Resort in Haad Thien and various national marine parks, providing opportunities for snorkeling, diving, and windsurfing across its scenic coastline.

Ko Pha-ngan is globally renowned for its world-famous Full Moon Party, which has placed it firmly on the Banana Pancake Trail. The island attracts visitors seeking both vibrant nightlife and tranquil escapes, with communities engaged in sustainable tourism and organic farming. It features prominently in popular culture, including Alex Garland's novel The Beach and various musical works. Local efforts promote environmental preservation, with initiatives to maintain green status and manage waste generated by hundreds of thousands of annual visitors.

The population was 13,700 as of 2012. Sea transportation via regular ferry and speedboat services remains the main way for visitors to travel to and from the island, connecting it to Ko Samui, Ko Tao, and the mainland. Motorbikes are a major means of local travel. While there have been attempts to construct an airport since 2012, delays and costs have hindered progress. The island receives approximately 458,000 visitors per year as of 2018, supporting a tourism-based economy alongside local agriculture.