The River Rother flows for 35 miles (56 km) through the English counties of East Sussex and Kent. Its source is near Rotherfield in East Sussex, and its mouth is on Rye Bay, part of the English Channel. Prior to 1287, its mouth was further to the east at New Romney, but it changed its course after a great storm blocked its exit to the sea. It was known as the Limen until the sixteenth century. For the final 14 miles (23 km), the river bed is below the high tide level, and Scots Float Sluice is used to control levels. It prevents salt water entering the river system at high tides, and retains water in the river during the summer months to ensure the health of the surrounding marsh habitat. Below the sluice, the river is tidal for 3.7 miles (6.0 km).

The river has been used for navigation since Roman times, and is still navigable by small boats as far as Bodiam Castle. It flowed in a loop around the northern edge of the Isle of Oxney until 1635, when it was diverted along the southern edge. Scots Float Sluice was built before 1723, when the engineer John Reynolds made repairs to it, and later extended it, to try to keep the channel clear of silting, but it was criticised by John Rennie in 1804, as it was inconvenient to shipping. The river became part of a defensive line to protect England from the threat of invasion by the French in the early 1800s, when its lower section and part of the River Brede formed a link between the two halves of the Royal Military Canal. Scots Float Sluice was again rebuilt in 1844. Some 31 square miles (80 km²) of the valley were inundated by floodwater in 1960, which resulted in the Rother Area Drainage Improvement Scheme being implemented between 1966 and 1980. The river banks were raised, and 20 pumping stations were installed.

The river has been managed by a number of bodies, including the Rother Levels Commissioners of Sewers, the Rye Harbour Commissioners, and the Board of Conservators for the River Rother. After the passing of the Land Drainage Act 1930, it was managed by the Rother and Jury's Gut Catchment Board, the Kent River Board, the Kent and Sussex River Authorities, the National Rivers Authority and finally the Environment Agency. It is unusual, in that while it is under the jurisdiction of the Environment Agency, it has been a free river since 1826, and so no licence is required to use it. Management of the levels adjacent to the river is undertaken by the Romney Marshes Area Internal Drainage Board. The Rother passes by or near the villages of Etchingham, Robertsbridge, Bodiam, Northiam, and Wittersham.

The modern name of the river is comparatively recent, probably dating from around the sixteenth century. It is derived from the village and hundred of Rotherfield, located where the river rises. The name Rotherfield is possibly derived from the Old English Hrytheran-felda, meaning 'open land of the cattle'. Prior to being called the Rother, it was known as the Limen throughout its length. This is a Celtic word meaning 'river'. In several Anglo-Saxon charters, it is suffixed with -ea, appearing as Limenea, where the suffix also means 'river', but in Old English. In the Anglo-Saxon Chronicle it is called the River Lympne or Lymne. During the thirteenth century, it was known as the River of Newenden.

The Rother rises in the High Weald of Sussex, at around 490 feet (150 m) above ordnance datum (AOD), and descends rapidly. It is joined by the River Dudwell at Etchingham and the River Darwell at Robertsbridge, and by the time it reaches Udiam, it is only 7 feet (2 m) AOD. Average annual rainfall in the High Weald is 35 inches (900 mm), and most of the underlying geology is impermeable, resulting in rain rapidly reaching the river and flowing down to the sea. The river valley is thus prone to winter floods, while during the summer months, the flow can be quite low in dry periods, as there are few groundwater aquifers. Between Udiam and Bodiam, the bed of the river drops below sea level, and the lower river flows slowly. The surrounding land is crossed by networks of canals and ditches, which are pumped into the river during the winter to drain the land. During the summer, water is transferred in the other direction, to manage the habitat of the marshland.

Scots Float sluice, some 3.7 miles (6 km) from the mouth of the river, is used to control levels. It is named after Sir John Scot(t), who enlarged a harbour on the site around 1480. The river below it is tidal, and it is closed as the tide rises, to prevent salt water passing up the river. During dry years, the sluice may be kept closed for most of the summer, as the water is used to maintain the marsh environment. A navigation lock bypasses the sluice. If heavy rainfall coincides with a high tide, where outflow is tide-locked, the river above the sluice to Bodiam acts as a huge holding reservoir for flood water, and is managed as such. In times of high flow, water is also pumped from the river at Robertsbridge into Darwell Reservoir, which can hold 167 million cubic feet (4730 Ml) of water. It covers an area of 156 acres (63 ha) and was built between 1937 and 1949. Since the 1980s, its output has been taken by pipeline to Beauport Park, from where it provides a public water supply for Hastings.

Near its mouth, the River Rother no longer follows its ancient course, as it once flowed across Romney Marsh and joined the sea at Dungeness. It is widely asserted that in 1287 a hurricane, known as the Great Storm, caused large quantities of shingle and mud to be deposited on the port of Romney and the mouth of the river. The water from the river created a new channel, joining the River Brede and the River Tillingham near Rye, where the combined rivers flow into the sea. However, Tatton-Brown has argued that patterns of occupation on Romney Marsh suggest that the change of route took place at least a century before that date. Rye became part of the Cinque Ports in the thirteenth century, and although it is situated some distance from the sea, its harbour is still visited by commercial shipping and has a fleet of fishing boats.

The river is known to have been used for shipping in Roman times, when it was navigable to Bodiam and possibly further upstream. There are records of small boats reaching Etchingham during Saxon and Norman periods. Stone for building Bodiam Castle was transported along the river in the fourteenth century, and iron was shipped from Newenden or Udiam in the sixteenth century. A century later, an iron store was erected at Udiam. Maytham Wharf served Rolvenden, while Tenterden was served by Small Hythe.