A clay court is one of the types of tennis court on which the sport of tennis is played. Clay courts are built on a foundation of crushed stone, particularly limestone, shale, and other aggregate, with a thin layer of fine clay particles on top. Clay courts are more common in Continental Europe and Latin America than in North America, Asia-Pacific, or Britain. The only Grand Slam tournament that uses clay courts is the French Open.

Clay courts come in the more common red clay, a distinctive color obtained from the superior layer of thin crushed brick, and the slightly harder green clay, which is actually crushed metabasalt. Although slightly less expensive to construct than other types of tennis courts, clay requires much maintenance: the surface must be watered and rolled regularly to preserve texture and flatness, and brushed carefully before and during each match.

Clay courts, although now commonly associated with continental Europe, were the creation of the English tennis player William Renshaw (a seven times Wimbledon champion) and his twin brother Ernest. The Renshaws had built a grass court at their summer home on the French Riviera, but found it unable to withstand prolonged exposure to heat and sunlight. To remedy this, in 1890 William Renshaw decided that the court should be covered with a fine powder that would protect it from the sun's rays. This powder was sourced from discarded terracotta discovered at a small ceramics factory in Vallauris, near Cannes, where the brothers regularly purchased decorative pots for their garden. Success with the new surface came quickly: after only a year or two, it was reported that 104 clay courts had been built in Cannes alone. Brick powder soon replaced the Vallauris terracotta pots, as the factory could not keep up with this eruption in demand.

Courts covered in a top layer of brick dust thereafter became the norm in many Mediterranean and South American countries, as they are not only durable in summer but do not need to be tended and mowed, unlike grass courts (although they do require regular maintenance involving rolling, "dragging", and occasional watering). Yet further improvements were still imperative, as the bounce on these courts was often irregular, the surface dried slowly after rain had fallen, and the courts could not be used in winter months for fear of damage. In 1909, the newly-established Leicestershire firm En-Tout-Cas (translated roughly into English as "in all conditions") came up with a prototype court made from crushed brick and burnt shale that could drain much more quickly than standard clay courts. These proved very popular once offered as a product, and by the 1920s the company was responsible for laying clay courts for the Davis Cup and at the French, British and Canadian Championships. During the following decade the American engineer H. A. Robinson went still further, creating a surface based on classic clay augmented with green piled metabasalt rock, which he hoped would help deliver a more consistent bounce. This became known as Har-Tru, derived from Robinson's initials and the 'true' nature of the bounce experienced on the court. First used in Hagerstown, Maryland, in 1932, green Har-Tru clay soon became the preferred surface for American clay court tournaments. In later years, the Har-Tru company would go on to develop all-weather clay surfaces at its factory in Charlottesville, Virginia, that could be played on throughout the entire year.

Almost all red clay courts are now made not of natural clay but of crushed brick that is packed to make the court, with the top most layers consisting of finely crushed loose particles. Such courts are most common in Europe and Latin America. The exact color of the surface varies with the composition of the bricks used, and can appear from a light yellow to a deep red.

Natural clay courts are rare because this type of surface does not absorb water easily and takes two to three days to dry. A good example of natural red clay can be seen at the Frick Park Clay Courts in Pittsburgh, a public facility of six red clay courts that has been in continual use since 1930.

Sports surface providers such as En-Tout-Cas still offer red clay surfaces with a coarser top layer to improve drainage. These courts are particularly popular in southern Australian states such as Victoria, where the phrase 'En-Tout-Cas' is a common generic shorthand for a clay court.

In Britain, where En-Tout-Cas is based, most red clay courts have a surface dressing of burnt shale (with or without brick) for drainage purposes. However, shale courts are now comparatively rare, having been steadily replaced in many clubs by all-weather synthetic surfaces (including artificial clay). The few courts that remain tend to be found in industrial areas such as central Scotland and Lancashire, as shale – known as blaes in Scotland – is a constituent element of colliery waste and thus can be easily and cheaply sourced in such locations. Shale 'clay' courts tend to be a lighter shade of red than their European counterparts, and the gritty texture of the surface means that the ball does not slow down as quickly as it does on powdery continental clay.