In geography, a floodplain is an area of relatively level land that is inundated from time to time. A floodplain may be border a stream, lake or river, or may be a watercourse in its own right. It is often defined as contained the floodway, which normally is inundated during annual flooding (or less often, say, ten-year flooding), and the floodway fringe (which may be inundated during a "100-year flood", "500-year flood" or even "1250-year flood").

Floodplains may be extremely broad, as in the case of the Platte River flowing across the Great Plains, where the boundary between river and floodplain is not even clear, or quite narrow, as in the case of entrenched rivers such as the Snake River in the Snake River Canyon or Colorado River in the Grand Canyon. In unmodified drainage systems where the terrain is fairly flat and rainfall intermittent, a floodplain may take the place of a river entirely. Instead of a defined streambed, there is simply a broad flat area where water flows from time to time.

Floodplains generally contain unconsolidated sediments, often extending below the bed of the stream or river. These are accumulations of sand, gravel, loam, silt, and/or clay, and are often important aquifers, the water being drawn from them being pre-filtered compared to the water in the river or stream.

Floodplains can support particularly rich ecosystems, both in quantity and diversity. These are termed riparian zones or systems. A floodplain can contain 100 or even 1000 times as many species as a river. Wetting of the floodplain soil releases an immediate surge of nutrients: those left over from the last flood, and those that result from the rapid decomposition of organic matter that has accumulated since then. Microscopic organisms thrive and larger species enter a rapid breeding cycle. Opportunistic feeders (particularly birds) move in to take advantage. The production of nutrients peaks and falls away quickly; however the surge of new growth endures for some time. This makes floodplains particularly valuable for agriculture.

Markedly different species grow in floodplains than grow outside of floodplains. For instance, riparian trees (that grow in floodplains) tend to be very tolerant of root disturbance and tend to be very quick-growing, compared to non-riparian trees.

Historically, many towns, homes and other buildings have been built on floodplains where they are highly susceptible to flooding, for several reasons:

• This is where water is most available
• Floodplain land is usually the most fertile for farming
• Rivers represent cheap sources of transportation, and are often where railroads are located
• The flatter land is easier to develop than hill land

However, insurers are now hostile to insuring buildings built in floodways, and in the USA the federal government has become tired of bailing out towns and people due to flood damage. A number of whole towns have been completely relocated to remove them from the floodplain, such as English, Indiana. In Europe governements try to ban new activities from floodplains as new activities (i.e. new structures) hinder discharge during floods.

Geologically ancient floodplains are often represented in the landscape by terrace deposits. These are old floodplain deposits that remain relatively high above current deposits, and can indicate former courses of rivers and streams.