The River Wear (/ˈwɪər/ ^ⓘ, WEER) in Northern England rises in the Pennines and flows eastwards, mostly through County Durham, to the North Sea in the City of Sunderland. At 60 mi (97 km) long, it is one of the region's longest rivers. The Wear wends in a steep valley through the cathedral city of Durham and gives its name to Weardale in its upper reach and Wearside by its mouth.

The origin behind the hydronym Wear is uncertain but is generally understood to be Celtic. The River Vedra on the Roman Map of Britain may very well be the River Wear. The name may be derived from Brittonic *wejr (<*wẹ:drā), which meant "a bend" (cf. Welsh -gwair-). An alternative but very problematic etymology might involve *wẹ:d-r-, from a lengthened form of the Indo-European root *wed- "water". Also suggested is a possible derivation from the Brittonic root *wei-, which is thought to have meant "to flow". The name Wear has also been explained as being an ancient Celtic name meaning "river of blood".

It is possible that the Wear has the same etymology as the River Wyre in Lancashire, the Quair Water in Scotland, the Weser in Germany and the Vistula in Poland.

The Wear rises in the east Pennines, high on the moors of the Alston Block, an upland area raised up during the Caledonian orogeny. The Devonian-age Weardale Granite underlies the headwaters of the Wear and the whole Alston Block, but does not appear at outcrop but was surmised by early geologists, and subsequently proven to exist as seen in the Rookhope borehole. It is the presence of this granite that has retained the high upland elevations of this area (less through its relative hardness, and more due to isostatic equilibrium) and accounts for heavy local mineralisation, although it is considered that most of the mineralisation occurred during the Carboniferous period.

It is thought that the course of the River Wear, prior to the last Ice Age, was much as it is now as far as Chester-le-Street. This can be established as a result of boreholes, of which there have been many in the Wear valley due to coal mining. However, northwards from Chester-le-Street, the Wear may have originally followed the current route of the lower River Team. The last glaciation reached its peak about 18,500 years ago, from which time it also began a progressive retreat, leaving a wide variety of glacial deposits in its wake, filling existing river valleys with silt, sand and other glacial till. At about 14,000 years ago, retreat of the ice paused for maybe 500 years at the city of Durham. This can be established by the types of glacial deposits in the vicinity of Durham City. The confluence of the River Browney was pushed from Gilesgate (the abandoned river valley still exists in Pelaw Woods), several miles south to Sunderland Bridge (Croxdale). At Chester-le-Street, when glacial boulder clay was deposited blocking its northerly course, the River Wear was diverted eastwards towards Sunderland where it was forced to cut a new, shallower valley. The gorge cut by the river through the Permian Magnesian Limestone (Zechstein limestone) can be seen most clearly at Ford Quarry. In the 17th edition of Encyclopædia Britannica (1990), reference is made to a pre-Ice Age course of the River Wear outfalling at Hartlepool.

The upland area of Upper Weardale retains a flora that relates, almost uniquely in England, to the end of the last Ice Age, although it almost or entirely lacks the particular rarities that make up the unique "Teesdale Assemblage" of post-glacial plants. This may, in part, be due to the Pennine areas of Upper Weardale and Upper Teesdale being the site of the shrinking ice cap, or to the difference in the surface geology, with none of the 'sugar limestone' outcrops which in Teesdale are the home of many of those plants. The glaciation left behind many indications of its presence, including lateral moraines and material from the Lake District and Northumberland, although surprisingly few drumlins. After the Ice Age, the Wear valley became thickly forested, however during the Neolithic period and increasingly in the Bronze Age, were largely deforested for agriculture.

Much of the River Wear is associated with the history of the Industrial Revolution. Its upper end runs through lead mining country, until this gives way to coal seams of the Durham coalfield for the rest of its length. As a result of limestone quarrying, lead mining and coal mining, the Wear valley was amongst the first places to see the development of railways. The Weardale Railway continues to run occasional services between Stanhope and Wolsingham.

Mining of lead ore has been known in the area of the headwaters of the Wear since the Roman occupation and continued into the nineteenth century. Spoil heaps from the abandoned lead mines can still be seen, and since the last quarter of the twentieth century have been the focus of attention for the recovery of gangue minerals in present mining, such as fluorite for the smelting of aluminium. However, abandoned mines and their spoil heaps continue to contribute to heavy metal mineral pollution of the river and its tributaries. This has significance to fishing in times of low flow and infrastructure costs as the River Wear is an important source of drinking water for many of the inhabitants along its course.