On the territory of today’s village of Prosiek Late Stone Age sites were situated; a settlement with cannel ceramics and a settlement of Púchov´s culture. The village situated between the Prosiecky brook and the village of Sielnica was first mentioned in 1287; and named as Prosiek since 1352.
Old and foreign names of the village were: Prezeky (1352), Prozyk (1353), Prozek (1391), Prosýk (1808), Prosiek (1920); Hungarian Proszek.
In 1391 it belonged to yeoman families Joob and Kubínyi. In 1784, the Joobs occupied 20 houses with 171 residents and the Kubínyis 23 houses with 194 residents. In 1828, it was 58 houses and 536 residents. They were engaged in agriculture and it did not change much during the period of the First Czechoslovak Republic.
Guerrilla warfare and the Slovak National Uprising were supported by the village. Nižný Zádiel and Vyšný Zádiel: these two municipalities were first mentioned in 1735 as Zadiel (1735), Alsó Zadiél vel Bölcsháza, Nižnj Zadjl (1808), Zadiel (1773); Wissny Zadiel (1773). They belonged to yeoman families Andaházy, Joob, and Šlachta. In 1784, they had 13 houses and 62 residents. In the mid-19th century, they merged into one village named Prosiek.