A watchtower or guardtower (also spelt watch tower, guard tower) is a type of military/paramilitary or policiary tower used for guarding an area. Sometimes fortified, and armed with heavy weaponry, especially historically, the structures are built in areas of established control. These include military bases, cities occupied by military forces, prisons and more. A common equipment is searchlights.

It differs from a regular tower in that its primary use is military/policiary and from a turret in that it is usually a freestanding structure. Its main purpose is to provide a high, safe place from which a sentinel or guard may observe the surrounding area. In some cases, non-military towers, such as religious towers, may also be used as watchtowers.

Similar constructions include: observation towers, which are generally civilian structures, and control towers, used on airports or harbours.

The Romans built numerous towers as part of a system of communications, one example being the towers along Hadrian's Wall in Britain. Romans built many lighthouses, such as the Tower of Hercules in northern Spain, which survives to this day as a working building, and the equally famous lighthouse at Dover Castle, which survives to about half its original height as a ruin.

In medieval Europe, many castles and manor houses, or similar fortified buildings, were equipped with watchtowers. In some of the manor houses of western France, the watchtower equipped with arrow or gun loopholes was one of the principal means of defense. A feudal lord could keep watch over his domain from the top of his tower.

In southern Saudi Arabia and Yemen, small stone and mud towers called "qasaba" were constructed as either watchtowers or keeps in the Asir mountains. Furthermore, in Najd, a watchtower, called "Margab", was used to watch for approaching enemies far in distance and shout calling warnings from atop.

Scotland saw the construction of Peel towers that combined the function of watchtower with that of a keep or tower house that served as the residence for a local notable family.

Mediterranean countries, and Italy in particular, saw the construction of numerous coastal watchtowers since the early Middle Ages, connected to the menace of Saracen attacks from the various Muslim states existing at the time (such as the Balearic Islands, Ifriqiya or Sicily). Later (starting from the 16th century) many were restored or built against the Barbary pirates. Similarly, the city state of Hamburg gained political power in the 13th century over a remote island 150 kilometers down the Elbe river estuary to erect the Great Tower Neuwerk by 1310 to protect its trading routes. They also claimed customs at the watchtower protecting the passage.