The Fens or Fenlands in eastern England are a naturally marshy region supporting a rich ecology and numerous species. Most of the fens were drained centuries ago, resulting in a flat, dry, low-lying agricultural region supported by a system of drainage channels, man-made rivers (dykes and drains), and automated pumping stations. There have been unintended consequences to this reclamation, as the land level has continued to sink and the dykes have been built higher to protect it from flooding.

Fen is the local term for an individual area of marshland or former marshland. It also designates the type of marsh typical of the area, which has neutral or alkaline water and relatively large quantities of dissolved minerals, but few other plant nutrients.

The Fens are a National Character Area, based on their landscape, biodiversity, geodiversity and economic activity.

The Fens lie inland of the Wash, and are an area of nearly 1,500 sq mi (3,900 km²) in the south east of Lincolnshire, most of Cambridgeshire (which also includes parts of the old historic county of Huntingdonshire), and western-most parts of Norfolk and Suffolk.

Most of the Fens lie within a few metres of sea level. As with similar areas in the Netherlands, much of the Fenland originally consisted of fresh- or salt-water wetlands. These have been artificially drained and continue to be protected from floods by drainage banks and pumps. With the support of this drainage system, the Fenland has become a major arable agricultural region for grains and vegetables. The Fens are particularly fertile, containing around half of the grade 1 agricultural land in England.

The Fens have been referred to as the "Holy Land of the English" because of the former monasteries, now churches and cathedrals, of Crowland, Ely, Peterborough, Ramsey and Thorney. Other significant settlements in the Fens include Boston, Downham Market, King’s Lynn, Mildenhall, March, Spalding, and Wisbech.

The Fens are very low-lying compared with the chalk and limestone uplands that surround them – in most places no more than 10 metres (33 ft) above sea level. As a result of drainage and the subsequent shrinkage of the peat fens, many parts of the Fens now lie below mean sea level. Although one writer in the 17th century described the Fenland as entirely above sea level (in contrast to the Netherlands), the area now includes the lowest land in the United Kingdom. Holme Fen in Cambridgeshire, is around 2.75 metres (9 ft 0 in) below sea level. Within the Fens are a few hills, which have historically been called "islands", as they remained dry when the low-lying fens around them were flooded. The largest of the fen-islands was the 23-square-mile (60 km²) Kimmeridge Clay island, on which the cathedral city of Ely was built: its highest point is 39 metres (128 ft) above mean sea level.

Without artificial drainage and flood protection, the Fens would be liable to periodic flooding, particularly in winter due to the heavy load of water flowing down from the uplands and overflowing the rivers. Some areas of the Fens were once permanently flooded, creating lakes or meres, while others were flooded only during periods of high water. In the pre-modern period, arable farming was limited to the higher areas of the surrounding uplands, the fen islands, and the so-called "Townlands", an arch of silt ground around the Wash, where the towns had their arable fields. Though these lands were lower than the peat fens before the peat shrinkage began, the more stable silt soils were reclaimed by medieval farmers and embanked against any floods coming down from the peat areas or from the sea. The rest of the Fenland was dedicated to pastoral farming, fishing, fowling, and the harvesting of reeds or sedge for thatch. In this way, the medieval and early modern Fens stood in contrast to the rest of England, which was primarily an arable agricultural region.