An altarpiece is a painting or sculpture, including relief, of religious subject matter made for placing at the back of or behind the altar of a Christian church. Though most commonly used for a single work of art such as a painting or sculpture, or a set of them, the word can also be used of the whole ensemble behind an altar, otherwise known as a reredos, including what is often an elaborate frame for the central image or images. Altarpieces were one of the most important products of Christian art especially from the late Middle Ages to the era of Baroque painting.

The word altarpiece, used for paintings, usually means a framed work of panel painting on wood, or later on canvas. In the Middle Ages they were generally the largest genre for these formats. Murals in fresco tend to cover larger surfaces. The largest painted altarpieces developed complicated structures, especially winged altarpieces with hinged side wings that folded in to cover the main image, and were painted on the reverse with different simpler images. Often this was the normal view shown in the church, except for Sundays and feast days, when the wings were opened to display the main image. At other times visitors could usually see this by paying the sacristan.

Altarpieces with many small framed panels are called polyptychs; triptychs have a main panel, and two side ones. Diptychs, with only two equally sized panels, were usually smaller portable pieces for individuals. The predella is a row of much smaller scenes running below the main panel; often these showed narrative scenes related to the subject of the main image. They were only properly visible from close up, but the extra height allowed the main panels above to be clearly seen by the congregation, and any shutters to be opened and closed with less disturbance to other items on the altar.

Many altarpieces have now been removed from their church settings, and often from their elaborate sculpted frameworks, and are displayed as more simply framed paintings in museums and elsewhere.

In the first several centuries of large Christian churches being built, the altar tended to be further forward (towards the congregation) in the sanctuary than in the later Middles Ages (a position to which it returned in the 20th century) and a large altarpiece would often have blocked the view of a bishop's throne and other celebrants, so decoration was concentrated on other places, with antependiums or altar frontals, or the surrounding walls.

Altarpieces seem to have begun to be used during the 11th century, with the possible exception of a few earlier examples. The reasons and forces that led to the development of altarpieces are not generally agreed upon. The habit of placing decorated reliquaries of saints on or behind the altar, as well as the tradition of decorating the front of the altar with sculptures or textiles, preceded the first altarpieces. In the Romanesque period, painted altar frontals on panel seem to have been a common alternative location for paintings. Few survive, though small Catalonian churches preserved several, many now in the Museu Nacional d'Art de Catalunya in Barcelona. The development of altarpieces may have begun at the altars of side chapels, typically engaged with the wall behind, rather than at freestanding main altars.

Many early altarpieces were relatively simple compositions in the form of a rectangular panel decorated with series of saints in rows, with a central, more pronounced figure such as a depiction of Mary or Christ. An elaborate example of such an early altarpiece is the metal and enamel Pala d'Oro in Venice, extended in the 12th century from an earlier altar frontal. The appearance and development of these first altarpieces marked an important turning point both in the history of Christian art as well as Christian religious practice. It was considered a "significant development" because of its impact on the "nature and function of the Christian image...the autonomous image now assumed a legitimate position at the centre of Christian worship".

Painted panel altars emerged in Italy during the 13th century, and until the Renaissance were generally the largest and most significant type of panel painting. In the 13th century, it was not uncommon to find frescoed or mural altarpieces in Italy; mural paintings behind the altar served as visual complements for the liturgy. These altarpieces were influenced by Byzantine art, notably icons, which reached Western Europe in greater numbers following the conquest of Constantinople in 1204. During this time, altarpieces occasionally began to be decorated with an outer, sculptured or gabled structure with the purpose of providing a frame for individual parts of the altarpiece. Vigoroso da Siena's altarpiece from 1291 (pictured) is an example. This treatment of the altarpiece would eventually pave the way for the emergence, in the 14th century, of the polyptych.