The River Bann (from Irish: An Bhanna, meaning "the goddess"; Ulster-Scots: Bann Wattèr) is the longest river in Northern Ireland, its length, Upper and Lower Bann combined, being 129 km (80 mi). The total length of the River Bann, including its path through the 30 km (19 mi) long Lough Neagh is 159 km (99 mi). Another length of the River Bann given is 90 mi. The river winds its way from the southeast corner of Ulster to the northwest coast, pausing in the middle to widen into Lough Neagh. The River Bann catchment has an area of 5,775 km². The River Bann has a mean discharge rate of 92 m³/s. According to C. Michael Hogan, the Bann River Valley is a settlement area for some of the first human arrivals in Ireland after the most recent glacial retreat.

The river has played an important part in the industrialisation in Northern Ireland, especially in the linen industry. Today salmon and eel fisheries are the most important economic features of the river. The river is often used as a dividing line between the eastern and western areas of Northern Ireland, often labelled the "Bann divide". Towns, councils and businesses "west of the Bann" are often seen as having less investment and government spending than those to the east. It is also seen as a religious, economic and political divide, with Catholics and Irish nationalists being in the majority to the west, and Ulster Protestants and unionists in the majority to the east; and with the financial and industrial capital of Greater Belfast to the east with the west of the Bann being more agricultural and rural.

The Lough Neagh catchment drains 43% of the landmass of Northern Ireland, as well as some border areas in the Republic of Ireland, all in Ulster. The Rivers Agency manages the water level in the lough using a barrage at Toome. The current drainage scheme was engineered by Major Percy Shepherd and was enabled by the Lough Neagh and Lower Bann Drainage and Navigation Act (Northern Ireland) 1955. The levels are regulated between 12.45 metres and 12.6 metres above Ordnance Datum, as defined in the Lough Neagh (Levels) Scheme 1955 (as amended).

The Upper Bann rises at Slieve Muck in the Mourne Mountains, County Down and flows directly into Spelga Reservoir before continuing through a number of towns until after 64 kilometres (40 mi) it joins Lough Neagh at Bannfoot, County Armagh. This stretch is one of the most popular coarse fishing rivers in Europe. At Whitecoat Point near Portadown, it is joined by the Cusher River and connects with the now disused Newry Canal, which once gave access south to the Irish Sea.

Although the Upper Bann was officially abandoned as a navigation in 1954, it is still possible to navigate between Whitecoat Point and Lough Neagh. Entrance to the river from Lough Neagh is not easy, as the river is quite shallow at this point, and there are no navigation markers to assist. Once on the river, the jetties for the Bann Ferry are soon reached. It is possible to moor there, to visit the villages of Columbkille to the west or Bannfoot to the east. Bannfoot was originally called Charlestown after its builder, Charles Brownlow, who built it around 1830. Some 6 miles (10 km) from the mouth, the river is crossed by the M1 motorway. The bridge is the lowest on the navigable section, with an air draught of around 10 feet (3 m), although in strong northerly winds, water backs up in the river and the headroom is reduced.

From the bridge it is around 3 miles (5 km) to Portadown, and the river passes through pleasant rural scenery. Exploration of the town from the river is difficult, because water levels at Shillington Quay and at the jetty a little further upstream are very shallow. The river is crossed by the railway line from Portadown to Lurgan and then road bridges carrying the A3 road and the A27 road. The junction with the Cusher River and the entrance to the derelict Newry Canal is just over 1 mile (1.6 km) from the final bridge, and navigation of the river is possible for a short distance beyond that point.

Spelga Bridge, New Bridge, Eight Mile Bridge, Mill Bridge, Cavan Bridge, Mc Combs Bridge, Bannfield Bridge, Tirkelly Bridge, Roughan Bridge, Seafin Bridge, Kates Bridge, Circular Rd Bann Bridge, Mulligans Bridge, Balilievey Bridge, Lisnaree Bridge, Lindsays Bridge, A1 Bypass Bridge, Bannbridge Bridge, Milltown Bridge, Point Bridge, Tullylish Bridge, Gilford Bridge, White Bridge, Dynes Bridge, Bann Bridge 1838, Shillington Bridge 1970, Bann Railway Bridge, M1 Bridge 1967.

The Lower Bann flows from Lough Neagh at Toome to the Atlantic Ocean at Barmouth, located behind Portstewart Golf Club, between Portstewart and Castlerock. The Lower Bann is 64 kilometres (40 mi) long and is a canalised waterway, with five navigation locks, at Toome, Portna, Movanagher, Carnroe and Castleroe. The river is very popular with water sports enthusiasts, anglers and cruisers and has minimal commercial traffic. It acts as most of the border between County Antrim and County Londonderry. The only commercial port on the river is at Coleraine. Ships from Londonderry Port and the Port of Belfast transfer coal and scrap metal.