He has said he will launch an international fundraising campaign for the rebuild.
His comments come after two towers have been saved from “total destruction”.
He said: “Notre-Dame is our history, our literature, part of our psyche, the place of all our great events, our epidemics, our wars, our liberations, the epicentre of our lives.”
“Notre-Dame is burning, and I know the sadness, and this tremor felt by so many fellow French people.
“But tonight, I’d like to speak of hope too,” he said, announcing the launch of an fundraising campaign.
“Let’s be proud, because we built this cathedral more than 800 years ago, we’ve built it and, throughout the centuries, let it grow and improved it.
“So I solemnly say tonight: we will rebuild it together.”
The spire and roof of the landmark in France’s capital collapsed yesterday as flames ripped though the 850-year-old building.
Flames brought down the church spire that extended 315ft high.
The deputy mayor of Paris, Emmanuel Gregoire, said the cathedral had suffered “colossal damages”.
The cause of the fire is not yet clear but it could be linked to renovation works at the historic site.
A cathedral spokesman said the entire wooden interior was burning and likely to be destroyed.
More to follow…