The campanile of St. Mark's is an imposing square plan tower about 99 metres high, crowned by a spire that was once a lighthouse for shipping. It is the prototype of all the campaniles of the lagoon area. 

It was first built in the 12th century on the site of what was probably a watchtower and rebuilt in its current form early in the 16th centurywith the addition of a belfry and with the spire faced in copper and topped by a sort of rotating platform with a statue of the Archangel Gabriel which functioned as a weathercock. Of the five original bells only the largest remains. The others, now replaced, were destroyed when the tower collapsed in 1902. 

From the belfry loggia there is a spectacular bird's eye view of the city and the lagoon. Against the base of the campanile is the balcony built by Jacopo Sansovino between 1537 and 1549 and decorated with marbles and bronzes.