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Desalination is the artificial process by which salt water (generally sea water) is converted to fresh water. More generally, desalination is the removal of salts and minerals from a substance. It is possible to desalinate saltwater, especially sea water, to produce water for human consumption or irrigation, producing brine as a by-product.

Interest in desalination mostly focuses on cost-effective provision of fresh water for human use. Along with recycled wastewater, it is one of the few water resources independent of rainfall. As stress on the need for freshwater intensifies globally, desalination has become a key part of strategies for global water security. According to a 2019 review in Science of the Total Environment, around 95 million cubic meters per day of desalinated water is produced worldwide, and the demand for desalinated water is expected to grow significantly to help close the global water supply gap.

Due to its energy consumption, desalinating sea water is generally more costly than fresh water from surface water or groundwater, water recycling and water conservation. However, these alternatives are not always available and depletion of reserves is a critical problem worldwide. Desalination processes use either thermal methods (in the case of distillation) or membrane-based methods (e.g. in the case of reverse osmosis).

As of 2020, global desalination capacity stood at roughly 97 million m³/day from over 16,800 operating plants, with contracted projects pushing total potential capacity beyond 114 million m³ /day worldwide. In 2018, the global energy intensity of desalination was about 3 kWh/m³ (in 2018), improved by a factor of 10 from 20–30 kWh/m³ in 1970. Nevertheless, desalination represented about 25% of the energy consumed by the water sector in 2016. Key companies in the desalination industry include Acciona, Dow, Evoqua Water Technologies, Siemens AG, DuPont, Doosan Enerbility, Toray Industries Inc., and Xylem.