It is understood the attacker went after the officer while he was on patrol on the esplanade in front of the cathedral. The officer then used his weapon to shoot the attacker, who was taken to hospital.
The incident happened at around 4.30pm local time on Tuesday, and large numbers of police cars filled the area on the Ile de Cite island in the Seine River as passers-by were urged to avoid the scene.
Witnesses described a dramatic police operation in the tourist-filled area.
Lawrence Langner, a 73-year-old American, said he suddenly heard a commotion and two detonations like gunshots.
Journalist David Metreau, who said his office overlooks the square that fronts Notre-Dame, tweeted there were two blasts that sounded like shots. He posted a photo of a body on the ground.
Police said the operation was over about an hour after it started.
Paris remains under high security after a string of Islamic extremist attacks in recent years, including several targeting police officers and security personnel.
In April, an attacker opened fire on a police van on the Champs Elysees, killing one and seriously injuring two others. The attacker was shot dead by police.