The city hall said 10 people had jumped into the River Seine (photo) and had been rescued. An adult and a child who were trapped in the sinking boat had been saved by divers and were being resuscitated, it said.
Police gave few details but said two people were seriously injured in the incident.
State radio station France Info reported from the scene that everyone who had been on the boat was now accounted for. It said the adult and child rescued from the boat were in a critical condition and were being treated nearby.
The incident took place in the heart of Paris, under the Pont de l'Archeveche which links the tip of the Ile de la Cite, where Notre Dame cathedral is located, to the Left Bank of the river.
The city hall said it did not know exactly what kind of boat was involved, although it was not a "bateau mouche", one of the huge, flat barge-like boats that carry hundreds of people up and down the river.
France Info said Transport Minister Dominique Bussereau had rushed to the scene.
An inquiry would be launched to determine the cause of the accident, the radio station said, adding it was not clear if the boat had hit the bridge or a bateau mouche that was now moored nearby, "La Besogne".
Dozens of tourist boats ply the Seine day and night and accidents are extremely rare.