San Marco is one of the six sestieri of Venice, lying in the heart of the city. San Marco also includes the island of San Giorgio Maggiore. Although the district includes Saint Mark's Square, that was never administered as part of the sestiere.

The small district includes many of Venice's most famous sights, including St Mark's Square, Saint Mark's Basilica, the Doge's Palace, Harry's Bar, the Palazzo Dandolo, San Moisè, the La Fenice theatre, the Palazzo Grassi and the churches of San Beneto, San Fantin, Santa Maria del Giglio, San Maurizio, San Moisè, Santo Stefano, San Salvador, San Zulian and San Samuele.

The area is densely built and was the location of Venice's government. It is now heavily touristed and there are many hotels, banks and expensive shops.