The US President took to Twitter to urge French fire chiefs to “act quickly” and suggested the use of flying water tankers to douse the flames.
Flames burst through the roof of the cathedral, which had scaffolding around it and the fire quickly engulfed the spire
Mr Trump tweeted: “So horrible to watch the massive fire at Notre Dame Cathedral in Paris.
“Perhaps flying water tankers could be used to put it out. Must act quickly!”
Firefighters have cleared the area around the cathedral, which marks the very centre of the city.
Mayor Anne Hidalgo tweeted: “A terrible fire is underway at Notre-Dame Cathedral in Paris.”