A smoke detector or smoke alarm is a device that detects smoke and issues an alarm to alert nearby people that there is a potential fire. A household smoke detector will typically be mounted in a disk-shaped plastic enclosure about 150 mm in diameter and 25 mm thick, but the shape can vary by manufacturer.

Because smoke rises, most detectors are mounted on the ceiling or on a wall near the ceiling and, to avoid the nuisance of false alarms, are mounted away from kitchens. To increase the chances of waking sleeping occupants, most homes have at least one smoke detector near any bedrooms; ideally, in a hallway, as well as in the bedroom itself.

Smoke detectors are usually powered by one or more batteries, but some can be connected directly to the power mains. Detectors that are directly connected to the main's often have a battery as a power supply backup in case the main's power fails. In either case, it is usually necessary to replace the batteries once a year to ensure appropriate protection if alkaline or carbon-zinc batteries are used.

Most smoke detectors work either by optical detection (photoelectric) or by physical process (ionization), but some of them use both detection methods to increase sensitivity to smoke. Smoke detectors may operate alone, may be interconnected to cause all detectors in an area to sound an alarm if one is triggered, or may be integrated into a fire alarm or security system. Smoke detectors with flashing lights are available for people who are deaf or hearing impaired, although recent research suggests that their waking effectiveness is poor.

A smoke detector cannot detect carbon monoxide to prevent carbon monoxide poisoning, unless it has an integrated carbon monoxide detector. These are also available as a separate detector. Most companies that manufacture smoke detectors also manufacture carbon monoxide detectors.

The smoke detector is one of three items of fire safety apparatus that are recommended for homes and that can be installed by the consumer. The second is a fire extinguisher, and the third is a fire blanket, a section of fire retardant cloth, normally 1 m² (3 ft x 3 ft), that can be applied to a small fire to smother it.