The woman, in her 20s, who was studying in Toronto, climbed on to a pillar overlooking the Canadian side of the falls and slipped over, police said.
She then fell into the Niagara River about 80ft (24m) up from the edge of the falls and was swept over.
The Niagara Parks Police described the death as a tragic accident.
As of Monday morning, the woman's body had not been recovered. Officials said they were working with the Japanese consulate to notify her family.
"The public is reminded that climbing over this wall is clearly dangerous and is prohibited by the Niagara Parks Act and its regulations," Niagara Parks Police Chief Doug Kane warned.
"Millions of visitors have viewed the scenic Niagara falls and the Niagara River gorge while respecting the safety wall without incident."
The accident, which occurred about 20:30 local time (00:30 GMT) on Sunday at Table Rock near the Horseshoe Falls on the Canadian side of Niagara Falls, was captured on CCTV.
The woman, who was visiting with a friend, had been taking a photograph, then climbed on to a pillar holding an umbrella before standing up for a clearer view, police said.
She lost her footing as she tried to climb down from a block pillar and tumbled into the fast-moving river, police said.