A fire brick, firebrick, or refractory brick is a block of refractory ceramic material used in lining furnaces, kilns, fireboxes, and fireplaces.

A refractory brick is built primarily to withstand high heat, but should also usually have a low thermal conductivity to save energy. Usually dense firebricks are used in applications with extreme mechanical, chemical, or thermal stresses, such as the inside of a wood-fired kiln or a furnace, which is subject to abrasion from wood, fluxing from ash or slag, and high temperatures. In other, less harsh situations, such as a natural gas fired kiln, more porous bricks are a better choice. They are weaker, but they are much lighter, easier to form, and insulate far better than dense bricks. In any case, firebricks should not spall under rapid temperature change, and their strength should hold up well during rapid temperature changes.

To make firebrick, fireclay is baked in the kiln until it is partly vitrified, and for special purposes may also be glazed. Fire bricks usually contain 30-40% aluminum oxide or alumina and 50% silicon dioxide or silica. They can also be made of chamotte and other materials. For bricks of extreme refractory character, the aluminum oxide content can be as high as 50-80% (with correspondingly less silica), and silicon carbide may also be present. The standard size of fire-brick is 9 x 4.5 x 2.5 in. (228 mm x 115 mm x 64 mm)

The silica firebricks that line steel-making furnaces are used at temperatures up to 1650C (3000F), which would melt many other types of ceramic, and in fact part of the silica firebrick liquefies. HRSI, a material with the same composition, is used to make the insulating tiles of the space shuttle.

A range of other materials find use as firebricks for lower temperature applications. Magnesium oxide is often used as a lining for furnaces.