The Kapuas River is a river in the Indonesian part of Borneo island, at the geographic center of Maritime Southeast Asia. At 1,143 kilometers (710 mi) in length, it is the longest river in the island of Borneo and the longest river in Indonesia and one of the world's longest island rivers. It originates in the Müller mountain range at the center of the island and flows west into the South China Sea creating an extended marshy delta. The delta is located west-southwest of Pontianak, the capital of the West Kalimantan province. This Kapuas River should be distinguished from another Kapuas River, which starts on the other side of the same mountain range in central Borneo but flows to the south, merging with the Barito River and discharging into the Java Sea.

Indonesia, officially the Republic of Indonesia, is a country in Southeast Asia and Oceania between the Indian and Pacific oceans. It consists of over 17,000 islands, including Sumatra, Java, Sulawesi, and parts of Borneo and New Guinea. Indonesia is the world's largest archipelagic state and the 14th-largest country by area, at 1,904,569 square kilometres. With over 279 million people, Indonesia is the world's fourth-most-populous country and the most populous Muslim-majority country. Java, the world's most populous island, is home to more than half of the country's population.