An analysis of samples taken from the waterway showed higher than authorized levels of E. coli bacteria.
"As a precautionary measure and to protect the health of the athletes, the decision has once again been taken to cancel all the swimming races scheduled for today," the organizers said in a statement.
It was the third such cancellation in a month. A paratriathlon test event was canceled on Saturday, while the Open Water Swimming World Cup was abandoned earlier in August after heavy rain caused overflows of untreated waste into the Seine.
Swimming in the Seine in 2024?
Paris officials are hoping to clean up the picturesque river so that open-air swimming events can take place there during next year's Olympic Games, which kick off on July 26.
To reach that goal, authorities have invested heavily in water-management projects geared towards reducing pollution from storm runoff.
Tony Estanguet, the head of the Paris 2024 organizing committee, stressed there was no "Plan B" for the swimming events.
"There is no solution to move the event, the triathlon and open water swimming will be held in the Seine next year," he said. "The objective was to make the Seine swimmable in 2024."
