According to chronicles, Zaslawye was founded in 985 by Vladimir the Great who sent his wife Rogneda to live here with their son Izyaslav of Polotsk, the founder of the princely house of Polatsk. The town's current name derives from Izyaslav's name.
In the early Middle Age the town was centre of the Duchy of Zaslawye. In the 11th century, the town was heavily fortified; much of its territory has been designated an archaeological reservation. There is also a modern outdoor statue of Rogneda and Izyaslav on the grounds.
During the period of Reformation, the town was a nest for followers of Calvinism and Socinianism
