Police and fire crews arrived at the scene around 10:20 a.m. after witnesses reported seeing a man climb over the retaining wall and deliberately jump into the river waters.
The man, believed to be in his 40s, went over the Horseshoe Falls and surfaced in the lower Niagara River basin adjacent the Journey Behind the Falls observation platform, Niagara Parks Commission Police said in a release.
"He waded ashore," said Platoon Chief Dan Orescanin of the Niagara Falls Fire Department.
"He must have gotten swept into an eddy, floated over there and was able to get out on his own.
"That's another stroke of luck," Orescanin said. "If he was in the main current, he would have been swept down river."
The man was swimming around when rescue crews located him. The rescuers rappelled down the steep and rocky gorge and pulled him in a basket back up the cliff.
"It was very difficult. Between the shale and the boulders, and everything is wet and slick. It's slimy," Orescanin said.
The man was brought to safety at about 12:16 p.m. as a crowd of Victoria Day tourists watched in shock.
About seven rescuers struggled to carry the basket up to a point where it could be lifted with ropes suspended from an aerial truck.
"We had to basically hand-carry him back up, a foot at a time, up the rope," Orescanin said.
Orescanin said the man was conscious and talking at first but got quiet. He appeared to have chest injuries, including broken ribs and a collapsed lung, Orescanin said.
Police said the man sustained life-threatening injuries. He was transported by an ORNGE air ambulance to a Hamilton General Hospital.
Hospital spokeswoman Agnes Bongers said later that the man was critically injured but was expected to survive. Authorities did not release the man's name.
Authorities don't believe Monday's plunge was a stunt.
"Based on witness statements and surveillance video, it doesn't appear in any way, shape, or form that this was anything other than a suicide attempt," Niagara Parks Police Sgt. Chris Gallagher told WIVB in Buffalo.
The last person to go over the Falls unaided and survive was a 30-year-old Canadian man in March 2009. In October 2003, a man from Michigan survived his plunge over the falls.
_ With files from The Associated Press.