Water mist is an extinguishing method in fire protection. Water mist is tiny water droplets (size from 10–200 microns) dispersed in the air as fog. The fog occurs in a nozzle when a jet of water is mechanically crushed or when two jets of water collide at an angle so that they crush each other. The micro-droplets in the water mist stay suspended in the air for quite some time and are often dragged with the air into the fire area. When the water mist hits flames, each drop expands 1,760 times and displaces oxygen so that the flames are suffocated.

Water mist systems were developed for the offshore industry and most oil installations in the North Sea use such technology.

Water mist as an extinguishing agent has the advantage that the quantities of water used are small, and no major water damage occurs. Water mist is therefore considered a suitable extinguishing agent in buildings with high cultural heritage value. Interventions are also minimal as a single water mist nozzle can cover a room of 25 m². The technology is also well suited to being retrofitted in older buildings, as water mist systems can be linked to existing wiring networks. New low-pressure technology makes it possible to create water mist down to 4 bar.

Water mist is suitable for automatic extinguishing systems that are triggered in the initial phase of a fire. In care homes where people live who cannot escape without help in the event of a fire alarm, water mist is considered particularly beneficial, as the mist takes the smoke down and creates better breathing conditions.

The Storting is the most famous building in Norway that has a water mist system. In the dense wooden housing development at the world heritage site Røros, another type of dry water mist system has been established in the attic to prevent the spread of fire. In the event of a fire, the fire service here must connect pressurized water to trigger the extinguishing system.