Witnesses say a man in his 40s climbed over a railing and leapt into the Niagara River about 30 feet above Horseshoe Falls at 10:20 a.m. He surfaced 180 feet below in the lower river basin.
Rescuers found him near the rocky shore and pulled him from the water about two hours later. They say he suffered a gash to his head and had injured his ribs.
They believe this was a failed suicide attempt.
Horseshoe Falls, on the Canadian side of the river, is the tallest of the three main falls, higher than the American Falls and Bridal Veil Falls.
The Associated Press contributed to this report.