Macron was responding after Sports Minister Amelie Oudea-Castera unveiled a rainwater treatment plant on the Marne, which flows into the Seine just east of Paris.
She declared that ensuring the two rivers are fit for swimming was "one of the challenges of the century".
On his X account, Macron expressed his satisfaction at this "key step towards making the Seine swimmable".
"The water in the Seine will be cold... but clean", he wrote. "I will be able to confirm it personally."
The Olympic open-water races and triathlon swims will be held on a scenic stretch of the river through the heart of Paris where last summer the bacteria levels passed the city's safe maximum.
The cleanliness of the river is highly dependent on the weather. Heavy rain washes pollution off the banks and also forces drains and waste pipes to back up into the river.
Storms just before events could lead to Olympic postponements or cancellations.
At the opening on Tuesday, Oudea-Castera pointed out there were 94 days to go to the Olympics.
"We are ready, on time," she said. "We will be ready to meet our legacy."
The plant on the Marne, designed to clean water, would handle "the equivalent of an Olympic swimming pool per hour," said Olivier Capitanio, President of the Val-de-Marne Department.
A huge cistern for overflow water at Austerlitz on the eastern fringe of central Paris has been designed to hold the equivalent of 20 Olympic swimming pools worth of water.
The measures to clean up the two rivers should also enable local residents to bathe in them, something they have not been able to do in the Marne since 1970 and in the Seine since 1923.
Macron is not the first French leader to say he will make the Seine clean enough to bathe in.
In 1990, when he was Paris mayor before becoming president, Jacques Chirac promised the river would "soon" be fit for swimming and said he would celebrate by taking a dip. He never did.
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