Castello is the largest of the six sestieri of Venice, Italy. From the thirteenth century onward, the district grew around a naval dockyard on what was originally the Isole Gemini, although there had been, since at least the 8th-century, small settlements of the islands of San Pietro di Castello (for which the sestiere is named). This island was also called Isola d'Olivolo.
The land in the district was dominated by Arsenale, then the largest naval complex in Europe, and by the monasteries in the north of the quarter. Napoleon closed the Arsenal and planned what are now the Bienniale Gardens, and still more recently the island of Sant'Elena has been created, and land drained at other extremities of the quarter.
Other prominent sites and buildings in Castello include:
- Scuola di San Marco
- Church of Santi Giovanni e Paolo (San Zanipoli)
- Scuola of San Giorgio degli Schiavoni
- Church of San Giorgio dei Greci
- Campo and Church of Santa Maria Formosa
- Church and Ospedale della Pietà
- Church of San Zaccaria