The Rio Negro (Portuguese: Rio Negro [ˈʁi.u neɡɾu]; Spanish: Río Negro [ˈri.o ˈneɣɾo] "Black River"), or Guainía as it is known in its upper part, is the largest left tributary of the Amazon River (accounting for about 14% of the water in the Amazon basin), the largest blackwater river in the world, and one of the world's ten largest rivers by average discharge. Despite its high flow, the Rio Negro has a low sediment load (5.76 million tonnes per year on average in Manaus).

The source of the Rio Negro lies in Colombia, in the Department of Guainía where the river is known as the Guainía River. The young river generally flows in an east-northeasterly direction through the Puinawai National Reserve, passing several small indigenous settlements on its way, such as Cuarinuma, Brujas, Santa Rosa and Tabaquén. After roughly 400 km (250 mi) the river starts forming the border between Colombia's Department of Guainía and Venezuela's Amazonas State. After passing the Colombian community of Tonina and Macanal the river turns southwest. Maroa is the first Venezuelan town the river passes. 120 km (75 mi) further downstream the river receives the Casiquiare canal from the left (north), forming a unique link between the Orinoco and the Amazon river basin. Henceforth the river is called Rio Negro.

The river now continues in a southeastern direction passing the Venezuelan town of San Carlos de Río Negro, its largest settlement on the river, and Colombia's San Felipe. In this stretch the river is constantly fed with tributaries from both sides, and it quickly grows in size creating large river islands, a common feature for all rivers in the Amazon basin. After forming the border between Colombia and Venezuela for 260 km (160 mi) the Rio Negro reaches the Piedra del Cocuy, an igneous rock formation from the Precambrian era, belonging to the Guyana Shield. Here the Tripoint of Colombia, Venezuela and Brazil is found in the middle of the river and it now completely enters Amazonas State, Brazil. After passing Cucuí, the river continues south, only temporarily turning west for several kilometers. In Missão Boa Vista the Içana River joins the Rio Negro and in São Joaquim the Uaupes River, the largest tributary of the Rio Negro, also enters from the right hand side. The Rio Negro now turns markedly towards the east, forming several rapids and small islets on its way. It then passes Sao Gabriel da Cachoeira an important commercial city. After several more rapids and imposing views of the Adormecida mountain chain the river leaves the Guyana shield it traversed in its upper and middle course.

After the Marié River enters the Rio Negro the river continues its eastward course forming many large islands and becoming very wide at several locations. It passes local communities such as Santa Isabel do Rio Negro. During the wet season, the river floods the country far and wide here, sometimes to a width of 30 km (19 mi), for long distances. During this season, from April until October, it is a succession of lagoons, full of long islands and intricate channels with a lot of water wildlife. Near Carvoeiro the last major tributary of the Rio Negro, the Branco River joins the Rio Negro and the river temporarily forms the border between the state of Roraima and Amazonas State, Brazil. The river now takes a more southeastern course, becoming again very wide in many stretches before reaching the biggest city on its course Manaus.

The Anavilhanas National Park, a 350,018 ha (864,910 acres) conservation unit that was originally an ecological station created in 1981, protects part of the Anavilhanas river archipelago in this part of the river. Below the archipelago, it meets the Solimões River to form the Amazon River, creating a phenomenon known as the Meeting of Waters.

List of the major tributaries of the Rio Negro (from the mouth upwards)

Left tributary	Right tributary	Length		Basin size		Average discharge
		km	mi	km	sq mi	m/s	cu ft/s
Rio Negro		2,362	1,468	719,216	277,691	30,640.8	1,082,070
Lower Rio Negro
Taruma-Açu		139	86	1,372	530	52.3	1,850
Cuieiras				3,441	1,329	144.2	5,090
Apuaú				3,799.1	1,466.8	153.6	5,420
	Puduari			3,411.5	1,317.2	128.6	4,540
Camanaú				11,832.2	4,568.4	408	14,400
	Jaú	400	250	18,896.6	7,296.0	869.2	30,700
Jauaperi		554	344	39,823.5	15,375.9	1,336.8	47,210
	Unini	530	330	27,433	10,592	1,501.4	53,020
Branco		1,430	890	190,789.2	73,664.1	5,400	190,000
Jufari		311	193	12,590.5	4,861.2	446.6	15,770
	Cauarés			7,332.3	2,831.0	296	10,500
Demini				39,769.6	15,355.1	1,357.9	47,950
	Cuiuni	400	250	11,776.1	4,546.8	491.6	17,360
Igarapé Adairá				3,294.9	1,272.2	101.1	3,570
	Ararirá			3,425	1,322	114.6	4,050
Ererê				3,251	1,255	115.7	4,090
Padauari				17,384	6,712	606.8	21,430
Daraá				3,053.3	1,178.9	114.9	4,060
	Urubaxi	250	160	6,855.9	2,647.1	311.6	11,000
	Aiuanã			4,590.7	1,772.5	185.2	6,540
	Uneiuxi			12,474.7	4,816.5	488.3	17,240
	Teá			6,365.9	2,457.9	201.4	7,110
Marauiá				6,712	2,592	255.3	9,020
Igarapé Inambu				4,618.8	1,783.3	140.6	4,970
Cauaburi				12,139.3	4,687.0	442.9	15,640
Middle Rio Negro
	Marié	800	500	25,378	9,799	1,226.8	43,320
	Curicuriari			14,202.2	5,483.5	916.8	32,380
	Uaupés	1,375	854	64,370.4	24,853.6	4,344.9	153,440
	Içana	696	432	35,675.3	13,774.3	2,278.9	80,480
	Xié			8,222	3,175	488.1	17,240
Upper Rio Negro
Casiquiare		354	220	42,478	16,401	2,575.8	90,960
	Guainía	617	383	28,899.5	11,158.2	2,432.7	85,910

Average discharge

Period	Discharge	Ref.
Encontro das Águas
1979–2015	30,640.8 m/s (1,082,070 cu ft/s)
1971–2000	29,615.7 m/s (1,045,870 cu ft/s)
Manaus 3°8′19.3704″S 60°1′39.522″W / 3.138714000°S 60.02764500°W / -3.138714000; -60.02764500
2015–2019	34,573 m/s (1,220,900 cu ft/s)
1980–2006	35,943 m/s (1,269,300 cu ft/s)
1973–1992	28,400 m/s (1,000,000 cu ft/s)
1971–2000	29,607.4 m/s (1,045,580 cu ft/s)
Paricatuba 3°4′53.2848″S 60°14′0.4812″W / 3.081468000°S 60.233467000°W / -3.081468000; -60.233467000
2008–2019	34,444 m/s (1,216,400 cu ft/s)
1971–2000	29,518.2 m/s (1,042,430 cu ft/s)
Barcelos 0°58′59.6964″S 62°51′32.3892″W / 0.983249000°S 62.858997000°W / -0.983249000; -62.858997000
1971–2000	19,545.2 m/s (690,230 cu ft/s)
Serrinha 0°28′57.4716″S 64°49′36.6492″W / 0.482631000°S 64.826847000°W / -0.482631000; -64.826847000
1997–2015	16,845.5 m/s (594,890 cu ft/s)
1980–2006	18,082 m/s (638,600 cu ft/s)
1971–2000	16,611.9 m/s (586,640 cu ft/s)
São Gabriel da Cachoeira 0°8′2.9616″S 67°5′38.2956″W / 0.134156000°S 67.093971000°W / -0.134156000; -67.093971000
1971–2000	13,150.7 m/s (464,410 cu ft/s)
São Felipe 0°22′11.8056″N 67°18′42.9948″W / 0.369946000°N 67.311943000°W / 0.369946000; -67.311943000
1997–2014	8,314.5 m/s (293,620 cu ft/s)
1980–2006	7,984 m/s (282,000 cu ft/s)
1971–2000	8,244.8 m/s (291,160 cu ft/s)
Cucuí 1°11′16.7316″S 66°50′23.46″W / 1.187981000°S 66.8398500°W / -1.187981000; -66.8398500
1997–2014	5,113.1 m/s (180,570 cu ft/s)
1980–2006	4,940 m/s (174,000 cu ft/s)
1971–2000	5,065.1 m/s (178,870 cu ft/s)

The river was named by the Spanish explorer Francisco de Orellana, who first came upon it in 1541. By the middle of the 17th century, Jesuits had settled along its banks in the midst of numerous tribes: Manau, Aruák, and Trumá Indians. After 1700 slaving along the river was common, and Native American populations were greatly diminished after contact with Eurasian diseases. This area was the filming location for Survivor:The Amazon in 2003.

While the name Rio Negro means Black River, its waters are similar in colour to strong tea, which is typical of blackwater rivers. The dark color comes from humic acid due to an incomplete breakdown of phenol-containing vegetation from sandy clearings. The river was named because it looks black from a distance.

Much has been written on the productivity of the Rio Negro and other blackwater rivers. The older idea that these are "hunger rivers" is giving way, with new research, to the recognition that the Rio Negro, for example, supports a large fishing industry and has numerous turtle beaches. If explorers did not find many Indigenous peoples along the Rio Negro during the 17th century, it is likely that their populations were reduced because of new infectious diseases and warfare rather than low river productivity.

Rio Negro has a very high species richness. About 700 fish species have been documented in the river basin, and it is estimated that the total is 800–900 fish species, including almost 100 endemics and several undescribed species. Among these are many that are important in the aquarium trade, including the cardinal tetra. As a result of the Casiquiare canal, many aquatic species are found both in the Rio Negro and Orinoco. Because the Casiquiare includes both blackwater and clear- to whitewater sections, only relatively adaptable species are able to pass through it between the two river systems.

The sixth season of Survivor, Survivor: The Amazon was filmed in Rio Negro in 2003.

• Goulding, M., Carvalho, M. L., & Ferreira, E. J. G. (1988). Rio Negro, Rich Life in Poor Water : Amazonian Diversity and Foodchain Ecology as seen through Fish Communities. The Hague: SPB Academic Publishing. ISBN 90-5103-016-9
• Saint-Paul, U., Berger, U., Zuanon, J., Villacorta Correa, M. A., García, M., Fabré, N. N., et al. (2000). "Fish communities in central Amazonian white- and blackwater floodplains," Environmental Biology of Fishes, 57(3), 235–250.
• Sioli, H. (1955). "Beiträge zur regionalen Limnologie des Amazonasgebietes. III. Über einige Gewässer des oberen Rio Negro-Gebietes." Arch. Hydrobiol., 50(1), 1-32.
• Wallace, A. R. (1853). A narrative of travels on the Amazon and Rio Negro, with an account of the native tribes, and observations on the climate, geology, and natural history of the Amazon Valley. London: Reeve.
• Wright, R. (2005). História indígena e do indigenismo no Alto Rio Negro. São Paulo, Brazil: UNICAMP & Instituto Socioambiental. ISBN 85-7591-042-6.