The Fly at 1,050 kilometres (650 mi), is the second longest river, after the Sepik, in Papua New Guinea. The Fly is the largest river in Oceania, the largest in the world without a single dam in its catchment, and overall ranks as the twenty-fifth largest river in the world by volume of discharge. It rises in the Victor Emanuel Range arm of the Star Mountains, and crosses the south-western lowlands before flowing into the Gulf of Papua in a large delta.

The Fly flows mostly through the Western Province, though for a small stretch it forms the boundary between PNG and the Indonesia province of Papua. This section protrudes slightly to the west of the 141°E longitude line. To compensate for this slight gain in territory for PNG, the border south of the Fly River is slightly east of the 141°E longitude line. As part of this deal, Indonesia has the right to use the Fly River to its mouth for navigation.

The principal tributaries of the Fly are the Strickland and the Ok Tedi.

The estuary of the Fly River is 56 km wide at its entrance, but only 11 km wide abreast Kiwai Island, which may be considered as being the river mouth. Above this island the river gradually contracts to a width of 1.6 kilometers or less.