A fen is a type of peat-accumulating wetland fed by mineral-rich ground or surface water. It is one of the main types of wetlands along with marshes, swamps, and bogs. Bogs and fens, both peat-forming ecosystems, are also known as mires. The unique water chemistry of fens is a result of the ground or surface water input. Typically, this input results in higher mineral concentrations and a more basic pH than found in bogs. As peat accumulates in a fen, groundwater input can be reduced or cut off, making the fen ombrotrophic rather than minerotrophic. In this way, fens can become more acidic and transition to bogs over time.

Fens can be found around the world, but the vast majority are located at the mid to high latitudes of the Northern Hemisphere. They are dominated by sedges and mosses, particularly graminoids that may be rarely found elsewhere, such as the sedge species Carex exilis. Fens are highly biodiverse ecosystems and often serve as habitats for endangered or rare species, with species composition changing with water chemistry. They also play important roles in the cycling of nutrients such as carbon, nitrogen, and phosphorus due to the lack of oxygen (anaerobic conditions) in waterlogged organic fen soils.

Fens have historically been converted to agricultural land. However, fens face a number of other threats, including peat cutting, pollution, invasive species, and nearby disturbances that lower the water table in the fen, such as quarrying. Interrupting the flow of mineral-rich water into a fen changes the water chemistry, which can alter species richness and dry out the peat. Drier peat is more easily decomposed and can even burn.