The River Erewash /ˈɛrɪˌwɒʃ/ is a river in England, a tributary of the River Trent that flows roughly southwards through Derbyshire and Nottinghamshire, forming the boundary between the two counties for much of its length. It rises near Kirkby-in-Ashfield, and passes close to Pinxton, Ironville, Langley Mill, Eastwood, Ilkeston, Trowell, Stapleford, Sandiacre, Toton and Long Eaton to reach the River Trent near Beeston.

It shares its valley with a railway line from Kirkby-in-Ashfield to Pye Bridge Junction, and the Erewash Valley line below the junction. The Pinxton Branch of the Cromford Canal runs parallel to the upper reaches, and below Ironville the main line of the abandoned Cromford Canal enters the valley. At Langley Mill, both the Erewash Canal and the abandoned Nottingham Canal fit into the valley, with the Nottingham Canal turning to the east at Trowell, and the Erewash Canal continuing through Long Eaton when the river turns to the east to reach its mouth.

The river passes through three nature reserves, at Erewash Meadows and Aldercar Flash to the north of Langley Mill, and the Attenborough Nature Reserve on the banks of the River Trent. They are managed by the Nottinghamshire Wildlife Trust and the Derbyshire Wildlife Trust. Historically, the river has supplied power to several water mills, none of which remain standing, apart from some of Stapleford Mill, which forms part of a social club established in 1922. The river is mentioned several times in the works of the writer D. H. Lawrence.

The approximate meaning of the name is not in doubt, but there is room for debate about the precise derivation and its connotations. Brewer gives the commonly accepted explanation that it comes from the Old English words irre ("wandering") and wisce ("wet meadow"). This is accepted by Kenneth Cameron, a leading placename expert and Derbyshire specialist, who interprets the name as "wandering, marshy river". Margaret Gelling, who specialises in seeking precise topographical equivalents for toponymic elements, confirms that wisce signifies a marshy meadow but gives only southern examples. She conjectures that there is an element, wæsse, perhaps Old English, that signifies very specifically "land by a meandering river which floods and drains quickly", and her examples are primarily Midland and northern. This seems to fit the Erewash perfectly. A good example of the meandering character of the river will be seen around Gallows Inn Playing Fields, Ilkeston, where rapid flooding and draining occur frequently. As it meanders through Toton and Long Eaton the river splits into two sections; the main course veers to the east and the relief channel flows over a low weir in a straight southerly direction. When there has been prolonged rainfall, the two waters are prone to bursting their banks and meet over the football pitches and the cricket pitch.

The Erewash rises on the south side of Kirkby-in-Ashfield, Nottinghamshire, close to a disused railway embankment to the south of Kirkby-in-Ashfield railway station. It flows to the south-west, and is joined by another stream which rises in Portland Park and flows to the north-west, passing under the railway line for Kirkby-in-Ashfield to Pye Bridge junction, where it joins the Erewash Valley line. It is culverted beneath another disused embankment, and turns westwards to the north of Kirkby Woodhouse. To the south of Bentinck Town it is crossed by the B6018 Park Lane, and is culverted under the M1 motorway. It is joined by Maghole Brook, which is also culverted under the motorway, a little further to the north. Pinxton in Derbyshire is to the north and Selston in Nottinghamshire is to the south. The county boundary follows the course of the river from the M1 crossing to its mouth. After the motorway, it is crossed by a railway bridge. It passes the basin at the end of the abandoned Pinxton Branch of the Cromford Canal, which shares the valley as the river turns to the south. At Pye Bridge the Erewash Valley line crosses over the river, after which it too shares the valley. The railway bridge is followed by the B600 Alfreton Road bridge. As it approaches Ironville, it is crossed by two railway embankments, now part of the heritage Midland Railway – Butterley, and continues to the east of the disused Cromford Canal to reach Erewash Meadows Nature Reserve.

Erewash Meadows is a large area of floodplain grassland and wetland. It is jointly owned by Derbyshire Wildlife Trust and Nottinghamshire Wildlife Trust, and includes 0.75 miles (1.21 km) of the Cromford Canal which is in water to the north of the ponds which form the central section of the reserve, known as Brinsley Meadows. The river provides habitat for fish and invertebrates, as well as a significant number of water voles. During the spring and summer months, the ponds and the watered section of the canal are populated by grass snakes, amphibians and dragonflies. Butterflies thrive on the wild flowers which inhabit the grasslands, and during the spring, the reserve is a haven for lapwings, beeding snipe, reed buntings and sedge warblers. Wading birds, wildfowl and hen harriers can be seen during the autumn months, and some of the wildfowl stay for the winter, including widgeon and teal. The site covers around 100 acres (42 ha). To the south is Aldercar Flash nature reserve, 22 acres (9 ha) of newly created ponds and reed beds, which house populations of little ringed plover, snipe, shelduck, reed warbler and water voles.

After the reserves, the river is crossed by the A610 road to pass between Langley Mill to the west and Eastwood to the east. It is joined by Nether Green Brook, which flows westwards to reach the Erewash's left bank. It is crossed by the A608 road to the east of Langley Mill railway station, loops around the site of the Heanor Milnhay sewage treatment works at Langley Mill, and is joined by Bailey Brook on its right bank. Severn Trent submitted a planning application in 2022 to abandon the treatment works and replace it with a pumping station and pipeline to allow the effluent to be treated elsewhere. To the south-west of Eastwood the river is crossed by a Grade II listed aqueduct, built in 1779 by the engineer John Varley to carry the Erewash Canal over the river. It is constructed of sandstone, and has three segmental arches. Gill Brook joins the river on its left bank, and it is crossed by the Grade II* listed Bennerley Viaduct. It was built in the late 19th century, and is one of only two wrought iron viaducts that survive in England. It was completed in 1887, and continued in use until 1968. It is 1,421 feet (433 m) long and carried the line around 59 feet (18 m) above the river. It probably survived because of the difficulty of cutting up wrought iron at the time it was abandoned, and was extensively restored in 2022.

The river is crossed by a railway line to the north of Ilkeston railway station, by the A6096 road which runs westwards into Ilkeston, and then by the railway line again. There is a short section where the river has been straightened to accommodate the railway, before it crosses the river twice more, with the A609 road crossing between the two railway bridges. On the northern edge of Stapleford the M1 motorway crosses, and the river passes between Sandiacre to the west and Stapleford to the east. Three more bridges follow, carrying the railway, the B5010 road, and the A52 Brian Clough Way. The river threads between the Erewash Canal and Toton Sidings, and is crossed by the railway for the final time to the north east of Long Eaton. It turns to the east to flow along the southern edge of Toton, and the main channel is supplemented by a relief channel to the south. Both are crossed by the A6005 road bridge, before they join again and enter Attenborough Nature Reserve to the south of Beeston, from where the Erewash flows into the River Trent.