NASA Blue Marble satellite image of the Fly River
|Countries||Papua New Guinea, Indonesia|
|- left||Strickland River|
|- right||Ok Tedi River|
|- location||Star Mountains, Papua New Guinea|
|- location||Gulf of Papua, Papua New Guinea|
|Length||1,050 km (652 mi)|
|Basin||76,000 km2 (29,344 sq mi) |
|- average||6,000 m3/s (211,888 cu ft/s)|
|Discharge elsewhere (average)|
|-||6,000 m3/s (211,888 cu ft/s)|
The Fly, at 1,050 kilometres (650 mi), is the second longest river in Papua New Guinea, after the Sepik. 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 25th-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.
Close to its mouth, the flow of the Fly River encounters a tidal bore, where an incoming high tide pushes water upstream until the changing of the tide. The range of this tidal bore is still undocumented.
The delta of the Fly River is over 100 km wide at its entrance, but only 11 km wide at the apex upstream of Kiwai Island. The delta contains 3 main distributary channels (the Southern, Northern, and Far Northern Entrances) that branch from a common point (the “apex”). The distributary channels are 5 to 15m in depth, separated by elongate, sand-mud islands that are stabilised by lush mangrove vegetation. The islands are eroded and rebuilt rapidly in the apex area, where they have lateral migration rates of up to 150 m/a, with slower rates for the more seaward islands. Upstream from the apex the river gradually narrows to a width of 1.6 km or less. The Fly Delta exhibits a distinctive funnel shape in plan view, attesting to the fundamental role of tidal currents in shaping the Delta's geomorphology. Mean spring tidal ranges are amplified within the delta, from around 3.5m at the seaward entrance of the distributary channels, reaching a peak of about 5m at the delta apex. Seismic profiles and radiometrically dated core samples indicate that the delta is prograding seawards at an average rate of about 6 m/a 
The river delta is studded with low and swampy islands covered with mangrove and nipa palm, with villages and cultivated areas on these islands. The land on both sides of the estuary is of the same character. The islands in the estuary are flat and covered with thick, fertile alluvial soil. The largest islands are Kiwai Island, Purutu Island, Wabuda Island, Aibinio Island, Mibu Island, and Domori Island. Kiwai, Wabuda and Domori are inhabited.
A list of the river delta islands is:
The inhabitants of the Fly River delta engage in agriculture and hunting. Coconut palm, breadfruit, plantain, sago palm, and sugar cane are grown.
The Fly was first discovered by Europeans in 1842 when Francis Blackwood commanding the corvette HMS Fly, surveryed the western coast of the Gulf of Papua. The river was named after his ship and he proclaimed that it would be possible for a small steam powered boat to travel up the mighty river.
In 1876, Italian explorer, Luigi D'Albertis, was the first person to successfully attempt this when he travelled 900 km into the interior of New Guinea, in his steamer, Neva. It was the furthest any European explorer had ever been into the island.
Both the Strickland and the Ok Tedi Rivers have been the source of environmental controversy due to tailings waste from the Porgera Mine and the Ok Tedi Mine, respectively. Sediment sampling and coring in the distributary channels of the Fly Delta had not detected copper concentrations significantly higher than background as of 1994  In 2008, Dr Ian Campbell, a former advisor to Ok Tedi Mining Limited, claimed that company data suggest significant portions of the Fly River floodplain are at a high risk from acid mine drainage.
- IUCN: The Fly River Catchment - A Regional Environmental Assessment, 1995
- Fragmentation and Flow Regulation of the World’s Major River Systems
- Frank Jacobs (March 13, 2012). "Who Bit My Border?". The New York Times.
- p.159, Barrie R. Bolton. 2009. The Fly River, Papua New Guinea: Environmental Studies in an Impacted Tropical River System. Elsevier Science. ISBN 978-0444529640.
- Harris, P.T., Baker, E.K., Cole, A.R., Short, S.A., 1993. A preliminary study of sedimentation in the tidally dominated Fly River Delta, Gulf of Papua. Continental Shelf Research 13, 441-472.
- Harris, P.T., Hughes, M.G., Baker, E.K., Dalrymple, R.W., Keene, J.B., 2004. Sediment transport in distributary channels and its export to the pro-deltaic environment in a tidally-dominated delta: Fly River, Papua New Guinea. Continental Shelf Research 24, 2431-2454.
- "Blackwood, Francis Price (1809 - 1854)". Australian Dictionary of Biography.
- Lightbody, Mark; Wheeler, Tony (1985). Papua New Guinea: a travel survival guide (3 ed.). Lonely Planet. p. 172. ISBN 0-908086-59-8.
- Harris, P.T., 2001. Environmental Management of Torres Strait: a Marine Geologist’s Perspective, in: Gostin, V.A. (Ed.), Gondwana to Greenhouse: environmental geoscience - an Australian perspective. Geological Society of Australia Special Publication, Adelaide, pp. 317-328.
- "PNG warned of environmental mining disaster". ABC News. Australian Broadcasting Corporation. 2008-09-06. Retrieved 2008-01-06.