The Blue Nile ( ; transliterated: ʿAbbai, but pronounced Abbai; ; transliterated: an-Nīl al-Āzraq) is a river originating at Lake Tana in Ethiopia. Sometimes in Ethiopia the river—especially the upper reaches—is called the Abbai.

The Abbai portion of the river is considered holy by many in Ethiopia, and is believed to be the Gihon river mentioned as flowing out of the Garden of Eden in Genesis 2. The Abay portion of the Blue Nile rises at Lake Tana and flows for some thirty kilometers before plunging over the Tis Issat Falls. The river then loops across northwest Ethiopia through a series of deep valleys and canyons into Sudan, by which point it is only known as the Blue Nile.

Although there are several feeder streams that flow into Lake Tana, the sacred source of the river is generally considered to be a small spring at Gish Abbai at an altitude of approximately . The Blue Nile much later joins the White Nile at Khartoum, Sudan and, as the Nile, flows through Egypt to the Mediterranean Sea at Alexandria. The Blue Nile is so-called because during flood times the water current is so high, it changes color to an almost black; since in the local Sudanese language the word for black is also used for the color blue.

The distance from its source to its confluence is variously reported as 1460 and 1600 km (907 and 1000 mi). The uncertainty over its length might partially result from the fact that it flows through virtually impenetrable gorges cut in the Ethiopian highlands to a depth of some 1500 m (4950 ft)—a depth comparable to that of the Grand Canyon in the United States.

The Blue Nile flows generally south from Lake Tana and then west across Ethiopia and northwest into Sudan. Within 30 km (18.6 mi) of its source at Lake Tana, the river enters a canyon about 400 km long. This gorge is a tremendous obstacle for travel and communication from the north half of Ethiopia to the southern half. The power of the Blue Nile may best be appreciated at Tis Issat Falls, which are 45 m (148 ft) high, located about 40 km (25 mi) downstream of Lake Tana. Despite the hazards and obstacles of the river, on January 29, 2005 Canadian Les Jickling and New Zealander Mark Tanner reached the Mediterranean Sea after an epic 148 day journey becoming the first to have paddled the Blue Nile from source to sea.

The flow of the Blue Nile reaches maximum volume in the rainy season (from June to September), when it supplies about two thirds of the water of the Nile proper. The Blue Nile, along with that of the Atbara River to the north, which also flows out of the Ethiopian highlands, were responsible for the annual Nile floods that contributed to the fertility of the Nile Valley and the consequent rise of ancient Egyptian civilization and Egyptian Mythology. With the completion in 1970 of the Aswan High Dam in Egypt, the Nile floods ended.