The Fens, also known as the 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 and 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 Lincolnshire, Cambridgeshire, and Norfolk.

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, March, Spalding, and Wisbech.