For the first time it is mentioned as a fortified city Snovs'k at the time of the Kyiv Rus'. Near to it in 1068 prince Svyatoslav with three thousand soldiers has won a 12-thousand army of nomads. The name occurs from the river of Snow ("sna" - to wash). In the middle of ХIV century the city was a part of Grand duchy Lithuanian. From ХVI century it received the modern name Sedniv. The national legend tells about a name origin: "The Crimean Tatars have attacked Snovs'k but couldn't destroy fortified city. Tatars named defenders of Snovs'k "syden'" - in Tatar language it means "malicious, brave".
In the end of ХVІІ century the city belonged to Cossack family Lyzohuby. In 1690 there was built well-known fortress which was the center of family manor. In 1654-1782 Sedniv was the center of the squadron of Chernihiv regiment.