A blockhouse is a small fortification, usually consisting of one or more rooms with loopholes, allowing its defenders to fire in various directions. It is usually an isolated fort in the form of a single building, serving as a defensive strong point against any enemy that does not possess siege equipment or, in modern times, artillery, air force or cruise missiles. A fortification intended to resist these weapons is more likely to qualify as a fortress or a redoubt, or in modern times, be an underground bunker. However, a blockhouse may also refer to a room within a larger fortification, usually a battery or redoubt.

The term blockhouse is of uncertain origin, perhaps related to Middle Dutch blokhus and 18th-century French blocus (blockade).

Blockhouses existed in ancient Greece, for example the one near Mycenae.

Early blockhouses were designed solely to protect a particular area by the use of artillery, and they had accommodation only for the short-term use of the garrison. The first known example is the Cow Tower, Norwich, built in 1398, which was of brick and had three storeys with the upper storeys pierced for six guns each. The major period of construction was in the maritime defence programmes of Henry VIII between 1539 and 1545. They were built to protect important maritime approaches such as the Thames Estuary, the Solent, and Plymouth. Often sited in pairs, the blockhouses were not built to a common design, but usually consisted of a stone tower and bastion or gun platform, which could be semi-circular, rectangular or irregular in shape. The last blockhouse of this type was Cromwell's Castle, built in Scilly in 1651.

Blockhouses were an ubiquitous feature in Malta's coastal fortifications built in the 18th century by the Order of St. John. Between 1714 and 1716, dozens of batteries and redoubts were built around the coasts of the Maltese Islands, while a few others were built in the subsequent decades. Almost every battery and redoubt had a blockhouse, which served as gun crew accommodation and a place to store munitions.

Many of the batteries consisted of a semi-circular or polygonal gun platform, with one or two blockhouses at the rear. The blockhouses usually had musketry loopholes, and in some cases were linked together by redans. Surviving batteries include Mistra Battery and Ferretti Battery, which both have two blockhouses, and Saint Mary's Battery and Saint Anthony's Battery, which have a single blockhouse.

Many of the redoubts consisted of a pentagonal platform with a rectangular blockhouse at the rear, although a few had semi-circular or rectangular platforms. Surviving redoubts with blockhouses include Baħar iċ-Ċagħaq Redoubt and Briconet Redoubt, both of which have a pentagonal plan. A few of the redoubts consisted of a single tower-like blockhouse without a platform, and were known as tour-reduits. Of the four tour-reduits that were built, only the Vendôme Tower survives today.

Originally blockhouses were often constructed as part of a large plan, to "block" access to vital points in the scheme. But from the Age of Exploration to the nineteenth century standard patterns of blockhouses were constructed for defence in frontier areas, particularly South Africa, New Zealand, Canada, and the United States.