An evaporative cooler (also swamp cooler, desert cooler, and wet air cooler) is a device that cools air through the simple evaporation of water. Evaporative cooling differs from air conditioning by refrigeration and absorptive refrigeration, which use vapor-compression or absorption refrigeration cycles. In the United States, the use of the term swamp cooler may be due to the odor of algae produced by early units. Air washers and wet cooling towers use the same principles as evaporative coolers, but are optimized for purposes other than cooling the air inside a building. For example, an evaporative cooler may be designed to cool coils of a large air conditioning system to increase its efficiency.

Evaporative cooling is especially well suited for climates where the air is hot and humidity is low. For example, in the United States, the western/mountain states are good locations, with swamp coolers very prevalent in cities like Denver, Salt Lake City, Albuquerque, El Paso, Tucson, and Fresno where sufficient water is available. In worldwide, Spain, Australia, Turkey, Middle Asia, Africa and Middle East contries are perfect places to use evaporative air cooling. Evaporative air conditioning is also popular and well suited to the southern (temperate) part of Australia. In dry, arid climates, the installation and operating cost of an evaporative cooler can be much lower than refrigerative air conditioning, often by 80% or so. However, evaporative cooling and vapor-compression air conditioning are sometimes used in combination to yield optimal cooling results. Some evaporative coolers may also serve as humidifiers in the heating season.

In moderate humidity locations there are many cost-effective uses for evaporative cooling, in addition to their widespread use in dry climates. For example, industrial plants, commercial kitchens, laundries, dry cleaners, greenhouses, spot cooling (loading docks, warehouses, factories, construction sites, athletic events, workshops, garages, and kennels) and confinement farming (poultry ranches, hog, and dairy) all often employ evaporative cooling. In highly humid climates, evaporative cooling may have little thermal comfort benefit beyond the increased ventilation and air movement it provides.