A river basin, also known as a drainage basin, is an area of land where all flowing surface water converges to a single point, such as a river mouth, or flows into another body of water, such as a lake or ocean. It is separated from adjacent basins by a perimeter, the drainage divide, made up of a succession of elevated features, such as ridges and hills. A basin may consist of smaller basins that merge at river confluences, forming a hierarchical pattern. Other terms for a drainage basin are catchment area, catchment basin, watershed, water basin, and impluvium.

A river basin encompasses all of the land surface dissected and drained by many streams and creeks that flow downhill into one another, and eventually into a river. The final destination is usually an estuary which eventually carries it to a larger body of water, such as a lake or ocean. River basins have typical features, including tributaries, confluences, watersheds, sources, and mouths. A river basin is an example of an open system because it is open to inputs from outside, such as precipitation, and is responsible for outputs from the system, such as river water into the sea and the evaporation of water to the atmosphere.

The boundaries of a drainage basin are determined by watershed delineation, a common task in environmental engineering and science. Drainage basins are important in ecology because as water flows over the ground and along rivers, it can pick up nutrients, sediment, and pollutants, which are transported towards the outlet of the basin and can affect the ecological processes along the way as well as in the receiving water source. Land use can also contribute to the volume of water reaching the river, in a similar way to clay soils.