A gatehouse, in architectural terminology, is a building enclosing or accompanying a gateway for a castle, manor house, fort, town or similar buildings of importance.

Gatehouses made their first appearance in the early antiquity when it became necessary to protect the main entrance to a castle or town. Over time, they evolved into very complicated structures with many lines of defense. Strongly fortified gatehouses would normally include a drawbridge, one or more portcullises, machicolations, arrow loops and possibly even murder-holes where stones would be dropped on attackers. In some castles, the gatehouse was so strongly fortified it took on the function of a keep, sometimes referred to as a "gate keep". Examples of such gate keeps can be found at Bodiam Castle and Beaumaris Castle. In the late Middle Ages, some of these arrow loops might have been converted into gun loops (or gun ports).

Sometimes gatehouses formed part of town fortifications, perhaps defending the passage of a bridge across a river or a moat, as Monnow Bridge in Monmouth. York has four important gatehouses, known as "Bars", in its city walls. One such is Micklegate Bar.