The Smestow Brook, sometimes called the River Smestow, is a small river that plays an important part in the drainage of Wolverhampton, South Staffordshire, and parts of Dudley in the United Kingdom, and has contributed to the industrial development of the Black Country. It is the most important tributary of the River Stour, Worcestershire and part of the River Severn catchment.

The name of the stream may be of Anglo-Saxon origin, although it was not written down before the 14th century in the Middle English forms Smetheslall and Smethestalle. As late as the 19th century, the name was still generally rendered Smestall in surveys of the county. It means ″place of the smiths″. The whole of this part of the West Midlands was famed for iron production from the Middle Ages onwards. The nearby Kinver Forest and Wyre Forest supplied charcoal for smelting and working iron before the Industrial Revolution. Both the lower Smestow and the Stour were lined with bloomeries and forges, their water used for cooling and later to power simple machinery.

Some local people maintain that the lower part of the stream, approximately from Wombourne, is properly called the River Smestow, while the upper section is the Smestow Brook. Certainly the lower Smestow is much more impressive since dredging and course alterations in the 1990s. In practice, however, both forms are used for the whole length of the stream, with Smestow Brook predominating. Similarly, the term Smestow Valley is sometimes reserved for the narrow section from Aldersley to Wightwick, although it can be used for the entire catchment, including the much wider plain south of Trescott. The Smestow itself created neither of these features: it simply flows through a landscape opened up by glaciation in the last Ice Age.

The Smestow is entirely non-navigable. However, its valley forms a natural north-south route of such importance that the Staffordshire and Worcestershire Canal was constructed as a substitute for a navigable river, the Smestow supplying it with water.

The Smestow flows through an important local nature reserve at Wolverhampton, and its lower course largely follows the conservation area associated with the canal, often through linear woodland, as well as small areas of wetland. Hence, it is home to a wide range of wildlife: if little is rare, the variety is great. Dredging and other works often disturb wildlife, but recolonisation is usually rapid. The reduction in pollution over the last three decades has allowed wildlife to diversify and flourish.

The Smestow Valley reserve claims no less than 170 species of bird as residents or visitors, with 55 species breeding locally. Winter sees the greatest variety with the regular blackbird and common chaffinch reinforced by visitors, like redwing, fieldfare, lesser redpoll, siskin, little grebe, common snipe, lapwing and golden plover. Raptors like the buzzard and sparrowhawk also hover.

Insect life is also rich and varied, with more than 20 kinds of butterfly seen on the reserve, including ringlet, common blue, holly blue, peacock, red admiral, painted lady, green-veined white, comma, gatekeeper, small skipper, large skipper, meadow brown, purple hairstreak, small heath and small copper and, rarely, brimstone and clouded yellow. There is also a great variety of damselflies and dragonflies.

The Smestow took its present shape as a result of the last Ice Age. Glacial action removed part of the low ridge, to the north of present-day Wolverhampton, which separates the River Trent and River Severn catchments, creating the Aldersley Gap. As a result, the Smestow was able to break through to the south, and was thus captured from the Trent by the Severn catchment.