Tourists in Venice put plastic bags over their legs and residents wore rubber boots as water rose to knee-high levels in many parts of the lagoon city on Thursday.
The median level of the Adriatic Sea swelled to about 1.4 meters (1.5 yards) above normal - the highest in nearly two years - sending water from the lagoon into St. Mark's Square and many narrow alleyways.
Wooden catwalks which are usually used to allow pedestrian passage over flooded areas were removed after the water rose above them, rendering them useless.
In some places, it was impossible to distinguish where canals ended and sidewalks began.
Much of Italy has been hit by heavy rain and strong winds over the past week.