Piazza San Marco, often known in English as St Mark's Square, is the principal public square of Venice, Italy, where it is generally known just as la Piazza. Almost all the other urban spaces in the city are called campi ("fields"). The Piazzetta is an extension of the Piazza towards San Marco basin in its southeast corner. The two spaces together form the social, religious and political centre of Venice and are referred to together. This article relates to both of them.

The Patriarchal Cathedral Basilica of Saint Mark, commonly known as St Mark's Basilica, is the cathedral church of the Patriarchate of Venice; it became the episcopal seat of the Patriarch of Venice in 1807, replacing the earlier cathedral of San Pietro di Castello. It is dedicated to and holds the relics of Saint Mark the Evangelist, the patron saint of the city.