Three masked robbers snatched a suitcase stuffed with millions of dollars in cash from a businessman who had just withdrawn the money from a bank in western Japan on Thursday, police said.
In the audacious daylight mugging, assailants set upon the 29-year-old victim as he dragged the case containing $380 million yen ($3.5 million) across a parking lot in downtown Fukuoka, a police spokesman and local media said.
He was sprayed in the face with what is thought to have been tear gas by the attackers, who made off in a car with the suitcase.
Few other details were initially available.
While Japan is still a cash-based society, it is unusual for someone to carry such a large sum, which would have weighed at least 38 kilogrammes (84 pounds).
The country has an enviable safety record and muggings and other forms of street crime, such as pickpocketing, are rare. But Japan does have a shadowy underworld run by 'yakuza' organised crime syndicates.
The case came after local media reports in December said thieves posing as police officers had walked off with more than $5 million of gold last year in Fukuoka after telling a group of men carrying it in briefcases to hand it over.
Police in Fukuoka have refused to confirm those reports, which claimed the theft took place in July, most likely involving organised crime gangs.