Sumatra (Indonesian: Sumatera) is an island in western Indonesia, westernmost of the Sunda Islands. It is the largest island entirely in Indonesia (two larger islands, Borneo and New Guinea, are shared between Indonesia and other countries) and the sixth largest island in the world at 473,481 km2 with a population of 50,365,538. Its biggest city is Medan with a population of 2,109,330.

Sumatra forming an elongated landmass spanned diagonal northwest — southeast axis. The Indian Ocean bordering west, northwest and southwest sides of Sumatra with islands chain of Simeulue, Nias and Mentawai bordering along southwestern coast. On the northeast side the narrow Strait of Malacca separating the island from Malay Peninsula, an extension of Eurasian continent. On the southeast the narrow Sunda Strait separating Sumatra with Java. The northern tip of Sumatra bordering the Andaman islands, while the lower eastern side there are the islands of Bangka and Belitung, Karimata Strait and Java Sea. The Bukit Barisan mountains that contains several active volcanoes formed the backbone of the island much on the southwest sides, while the northeast sides are outlying lowlands with swamps, mangroove and complex river systems. The equator crossed the island right in the center on West Sumatra and Riau provinces, making the climate of the island tropical hot and humid with lush tropical rain forest originally dominating the landscape.

Settler colonies began arriving in Sumatra around 500 BC, and several significant kingdoms flourished there. I Ching, a Chinese Buddhist monk, studied Sanskrit and spent four years of his life working in Palembang. The explorer Marco Polo visited Sumatra in 1292.

Sumatra has a huge range of plant and animal species but has lost almost 50% of its tropical rainforest in the last 35 years, and many species are Critically Endangered such as Sumatran Tiger, Sumatran Rhino and Sumatran Orangutan.

87% of Sumatrans are thought to be Muslim. The island is home to 22% of Indonesia's population.