Leszno (Polish pronunciation: [ˈlɛʂnɔ] ^ⓘ; German: Lissa, [ˈlɪsa]) is a historic city in western Poland, seat of Leszno County within the Greater Poland Voivodeship. It is the seventh-largest city in the province with an estimated population of 62,200, as of 2021.

Leszno is a former residential city of prominent Polish magnate families of Leszczyński and Sułkowski, including King Stanisław Leszczyński, under whose patronage it flourished to become one of the major economic and cultural centers of Greater Poland, as reflected in the variety of landmarks, especially of the Baroque period.

Located on a highway and railway line at about half the distance between the two main cities of western Poland, Poznań and Wrocław, Leszno is the largest city of south-western Greater Poland and a major location for industry, services and tourism. It is particularly well known as the location of an annual air show with attendance reaching tens of thousands of people from various countries, and as the home of Unia Leszno, one of the most accomplished motorcycle speedway clubs.

The city's unrecorded history dates to the 13th century. It was first mentioned in historical documents in 1393, when the estate was the property of a noble named Stefan Karnin-Wieniawa. The family eventually adopted the name Leszczyński (literal meaning "of Leszno"), derived from the name of their estate, as was the custom among the Polish nobility.

In around 1516, a community of Protestants known as the Unity of the Brethren (Unitas fratrum) were expelled from the Bohemian lands by King Vladislaus II and settled in Leszno. They were invited by the Leszczyński family, imperial counts since 1473 and who had converted to Calvinism. The arrival of the Bohemian Protestants, in addition to weavers from nearby Silesia, helped the settlement to grow.

In 1547 it became a town by a privilege according to Magdeburg Law granted by King Sigismund I of Poland. Leszno was a private town, administratively located in the Wschowa County in the Poznań Voivodeship in the Greater Poland Province. Leszno became the largest printing center in Greater Poland thanks to the activity of the Protestant community. Their numbers grew with the inflow of refugees from Silesia, Bohemia, and Moravia during the Thirty Years War.

In 1631, Leszno was vested with further privileges by King Sigismund III Vasa, who made it equal with the most important cities of Poland such as Kraków, Gdańsk and Warsaw. By the 17th century, the town had a renowned Gymnasium (school), which was headed by Jan Amos Komenský (known in English as Comenius), an educator and the last bishop of the Unity of the Brethren. Johann Heermann, a German-speaking poet, lived in Leszno from 1638 until his death in 1647. Between 1636 and 1639, the town became fortified and its area increased.

The era of Leszno's prosperity and cultural prominence ended during the Second Northern War, when the town was burnt down on 28 April 1656 by Polish forces faithful to King John Cassimir Vasa in retribution for letting in Swedish forces by protestant majority of burghers. Quickly rebuilt afterwards, it was set on fire again during the Great Northern War by Russian forces in 1707 and was ravaged by plague in 1709.