Although no surviving historical records deal directly with the founding of Venice, the history of the Republic of Venice traditionally begins with the foundation of the city at Noon on Friday, 25 March 421 CE, by authorities from Padua, to establish a trading-post in that region of northern Italy. The founding of the Venetian republic is also said to have been marked at that same event with the founding of the church of St. James.

According to tradition, the original population of the region consisted of refugees—from nearby Roman cities such as Padua, Aquileia, Treviso, Altino, and Concordia (modern Concordia Sagittaria), as well as from the undefended countryside—who were fleeing successive waves of Hun and Germanic invasions from the mid-second to mid-fifth centuries. This is further supported by documentation on the so-called "apostolic families", the twelve founding families of Venice who elected the first doge, who in most cases traced their lineage back to Roman families.