Niagara Parks Police Chief Doug Kane said the man "voluntarily entered into the water and refused medical assistance at the bottom."
He said the man was suffering from hypothermia and a head injury. Police were unable to get any information from the man because of the effects of the near-freezing water.
A witness called police shortly after 2 p.m. and told them a man had jumped into the Horseshoe Falls, one of Niagara Falls' three waterfalls. A short time later, the man was seen near the base of the falls.
Specially trained falls rescue crews tried to assist the man, but he swam away from them toward the middle of the river, Kane said.
A private helicopter was called in and got close enough to the victim so that wind from its blades forced him close enough to shore for rescuers to reach him.
"He wasn't cooperative," pilot Ruedi Hafen, owner of Niagara Falls Helicopter, told The Associated Press. "He didn't try to be helpful. We had a sling on him and he got out of it."
Hafen said the man, who had a "big gashing injury" on his head, was in the water for about 45 minutes and spent much of that time resisting attempts to help him.
A firefighter and another rescue worker swam about 60 yards from shore, grabbed hold of him and hauled him in, Niagara Fire Capt. David Belme said.
"He was on a suicide mission, I assume," Hafen said.
"I've never, in my career, seen someone so tough, swimming between the ice," he said.
The last person known to survive a plunge over the falls was a Michigan man, Kirk Jones, who climbed down a small embankment and jumped into the Niagara River on Oct. 20, 2003. Jones said he had been depressed and had been drinking.
At least 17 people - not including suicide attempts - are known to have gone over the falls.
Niagara Falls' three waterfalls are the American Falls, Horseshoe Falls and Bridal Veil Falls.
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Associated Press writers Ben Dobbin in Rochester, N.Y., and Michael Hill and Jessica Pasko in Albany, N.Y., contributed to this report.
