Kalisz (Polish: [ˈkaliʂ] ^ⓘ) is a city in central Poland, and the second-largest city in the Greater Poland Voivodeship, with 97,905 residents (December 2021). It is the capital city of the Kalisz Region. Situated on the Prosna river in the southeastern part of Greater Poland, the city forms a conurbation with the nearby towns of Ostrów Wielkopolski and Nowe Skalmierzyce.

Kalisz is one of the oldest cities in Poland and one of the two traditional capitals of Greater Poland (alongside Poznań). It has served as an important regional center in Poland since the Middle Ages as a provincial capital and notable royal city. It is one of the historical burial sites of medieval Polish monarchs and dukes of the Piast dynasty and the site of a number of significant events in Polish history as well as several battles. Since the 19th century it has been the center of an industrial district. It is the cultural, scientific, educational and administrative center of the eastern and southern Greater Poland region, and the seat of Roman Catholic Diocese of Kalisz.

There are many artefacts from Roman times in the area of Kalisz, indicating that the settlement had once been a stop of the Roman caravans heading for the Baltic Sea along the trade route of the Amber Trail. Calisia had been mentioned by Ptolemy in the 2nd century AD, although the connection is doubted by some historians who claim that the location mentioned by Ptolemy was situated in the territory of the Diduni in Magna Germania.

Archaeological excavations have uncovered early medieval settlement from the Piast dynasty period, c. 9th–12th centuries. Modern Kalisz was most likely founded in the 9th century as a provincial capital castellany and a minor fort. As part of the region of Greater Poland, i.e. the cradle of the Polish state, the town formed part of Poland since the country's establishment in the 10th century.

In 1106, Bolesław III Wrymouth captured the town, and made it a part of his feudal domain. Between 1253 and 1260 the town was incorporated according to the German town law called the Środa Śląska Law [pl] (after Środa Śląska), a local variation of the Magdeburg Law, and soon began to grow. One of the richest towns of Greater Poland, during the feudal fragmentation of Poland it formed a separate duchy ruled by a local branch of the Piast dynasty.

In 1264, the Statute of Kalisz was issued in the city by Bolesław the Pious. It was a unique protective privilege for Jews during their persecution in Western Europe, which in the following centuries made Poland the destination of Jewish migration from other countries. After Poland was reunited, the town became a centre of weaving and wood products, as well as one of the cultural centres of Greater Poland.

In 1282 the city laws were confirmed by Przemysł II of Poland, and in 1314 it was made the capital of the Kalisz Voivodeship by King Ladislaus the Short. Located roughly in the centre of Poland (as its borders stood in that era), Kalisz was a centre of trade. In 1331, the city was successfully defended by the Poles during a siege [pl] by the Teutonic Knights. Because of its strategic location, King Casimir III the Great signed a peace treaty with the Teutonic Order there in 1343. As a royal city, Kalisz managed to defend many of its initial privileges, and in 1426 a new town hall was built. The Polish Duke Mieszko III the Old was buried in Kalisz. In the 14th century, Jews of the town were attacked during epidemics by mobs which accused them of poisoning the wells of the town.

In 1574 the Jesuits came to Kalisz and in 1584 opened a Jesuit College, which became a centre of education in Poland; around this time, however, the importance of Kalisz began to decline somewhat, its place being taken by nearby Poznań.