The man "was observed to climb over retaining wall" around 04:00 (09:00 GMT) on Tuesday before falling over the cliff, police said on Twitter.
After searching beneath the falls, police found the man "sitting on rocks" with non-life threatening injuries.
The man, who police have not identified, was taken to hospital.
The man was near the brink of Horseshoe Falls - the largest of the three falls that make up Niagara Falls, officials say.
He was found on the side of the river near the Journey Behind the Falls observation platform, according to Buffalo News.