However, authorities said the blaze was quickly brought under control.
Witnesses told French television they saw smoke emanating from the spire just after midday, and recalled a devastating fire in 2019 at Notre Dame Cathedral in Paris that toppled its spire and collapsed its roof.
Rouen's 12th-century cathedral, which is considered a Gothic masterpiece, is widely beloved, not least because of a series of paintings by impressionist Claude Monet capturing its asymmetrical western facade.
It is also the tallest church in France, and among the tallest cathedrals in the world.
It is renowned for its three towers, each constructed in a different style.
Mayor Nicolas Mayer-Rossignol first posted on the social media platform X around midday that "the beginning of a fire" was underway "on the spire of Rouen cathedral".
Less than 90 minutes later, Stephane Gouezec, of the Seine-Maritime firefighters, said the source of the blaze had been located - some 120 metres high — and the fire contained.
Crews were working to ensure there were no remaining "hot spots", he added.
But he told reporters the risk of the flames spreading was low because the fire was in an area where there was mostly metal.
Gouezec said construction workers were the first to notice the fire and alerted authorities.
The cathedral was evacuated and a security perimeter put in place, according to regional officials.
Archbishop Dominique Lebrun told French media that the structure was expected to reopen on Friday.
Witnesses in Rouen were jittery since the memory of Paris' Notre Dame blaze is still etched in the national consciousness.
It caught fire five years ago, also while under renovation, and is scheduled to reopen in December after an unprecedented reconstruction effort.
The cause of that fire was deemed an accident.
