Slonim is a town in Grodno Region, in western Belarus. It serves as the administrative center of Slonim District. It is located at the junction of the Shchara and Isa rivers, 143 km (89 mi) southeast of Grodno. As of 2025, it has a population of 48,402.

Slonim has been known by several versions of its name as spoken by speakers of various languages: Сло́нім ( Belarusian), Słonim (Polish), Сло́ним (Russian). Slonim was first mentioned by scribes in chronicles in 1252 as Uslonim and in 1255 as Vslonim. According to one account (which is also considered by some to be an official one), the name of the city originates from the Slavic word zaslona (a screen), implying that the city once functioned as an outpost at the southern border of the Grand Duchy of Lithuania. ^{[citation needed]}

The earliest record is of a wooden fort on the left bank of the Shchara river in the 11th century, although there may have been earlier settlement.

The area was disputed between the Grand Duchy of Lithuania and Kievan Rus' in early history and it changed hands several times. In 1040, the Kievans won control of the area after a battle but lost Slonim to the Lithuanians in 1103. The Ruthenians retook the area early in the 13th century but were expelled by a Tatar invasion in 1241 and the town was pillaged. When, later in the year, the Tatars withdrew, Slonim became part of the Grand Duchy of Lithuania once again, in personal union with the Kingdom of Poland after the Union of Krewo of 1385.

Slonim was the location of one of many Roman Catholic churches where the priests had to know the Lithuanian language according to the Grand Duke of Lithuania Alexander Jagiellon in 1501. In 1532 King Sigismund I of Poland granted Slonim town rights. In 1558, King Sigismund II Augustus, in a privilege issued in Wilno (now Vilnius), established two two-week fairs.

In 1569, the Polish–Lithuanian union was transformed into a single state and Słonim became an important regional centre within the newly established Polish–Lithuanian Commonwealth. Administratively it was part of the Nowogródek Voivodeship. Thanks to the efforts of nobleman, statesman and Słonim starost Lew Sapieha, King Sigismund III Vasa renewed the town rights of Słonim and granted the city coat of arms, which included the Lis coat of arms of Sapieha. Also thanks to Lew Sapieha, from 1631 to 1685 the city flourished as the seat of the Lithuanian diet.

The wars had damaged Slonim, but in the 18th century, a local landowner, count Ogiński, encouraged the recovery of the area; a canal was dug to connect the Shchara with the Dnieper river, now known as the Oginski Canal. Ogiński also built a greater complex, combining an opera theater, a school of music and a school of ballet, and a printing house.

The Polish–Lithuanian Commonwealth was dismantled in a series of three "partitions" in the second half of the 18th century and divided among its neighbours, Prussia, Habsburg Austria and Russian Empire which took the largest portion of the territory. Slonim was in the area annexed by Russia in 1795. Administratively it was part of the Slonim Governorate until 1797, Vilna Governorate until 1801 and Grodno Governorate until World War I. In 1897 it was the fourth largest city of the governorate after the leading cities of Białystok, Grodno and Brześć.