Yes there are several “left luggage” services in Venice.
1) At Santa Lucia Railway Station. Opens early in the morning until midnight. It is not an expensive service at around 4 euros for 5 hours.
2) At Piazzale Roma, near the bus station at Piazzale Roma (Next to the Pullmam Bar). Doesn’t look very safe.
3) At Marco Polo Airport, on the ground floor (Arrivals).
4) At the Venice Cruise Terminal’s Marittima and San Basilio buildings (normally open on days when cruise ships are arriving and departing).
5) Stow Your Bags: This is a self-service luggage storage service with several locations throughout Venice, including one near Santa Lucia Train Station. They offer different locker sizes to accommodate your needs and you can book online or on arrival. Also this service is available at Saint Marc’s Square and Rialto Bridge located at Calle de l’Orso.
6) LuggageHero: This is another luggage storage service with several locations throughout Venice. They offer hourly and daily rates and you can book online or on arrival. They also have a network of local shops and hotels where you can store your luggage, including near Venezia Mestre Train Station, Venice Central Station, and St. Mark’s Basilica.
7) Bounce Luggage Storage – 24/7 Venice Station (www.usebounce.com – Calle Pesaro, 389 – 30121 Venezia (VE) – +44 1422 400776)