The Sepik River is the longest river on the island of New Guinea, and the second largest in Oceania by discharge volume after the Fly River. The majority of the river flows through the Papua New Guinea provinces of Sandaun and East Sepik, with a small section flowing through the Indonesian province of Papua.
The Sepik has a large catchment area, and landforms that include swamplands, tropical rainforests and mountains. Biologically, the river system is often said to be possibly the largest uncontaminated freshwater wetland system in the Asia-Pacific region. But, in fact, numerous fish and plant species have been introduced into the Sepik since the mid-20th century.