The waters in Venice's main canal turned fluorescent green on Sunday in the area near the Rialto bridge and authorities are seeking to trace the cause, Italy's fire department said.
The regional environmental protection agency has received samples of the altered waters and is working to identify the substance that changed their colour, the department said in a tweet.
The Venice prefect has called an emergency meeting of police forces to understand what happened and study possible countermeasures, the Ansa news agency reported.
The incident echoes recent episodes in Italy where environmental groups have been colouring monuments, including using vegetable charcoal to turn the waters of Rome's Trevi fountain black in a protest against fossil fuels.
However, unlike previous cases, no activist group has come forward to claim responsibility for what happened in Venice.