The study, released to local media on Wednesday, also showed rising adult addiction to the Internet and alcohol in a society long known for its tolerance of boozing and its love of technology.
"If something new becomes available, addiction will only rise," Susumu Higuchi, Japan's leading expert on addiction who headed the study, told local journalists according to the Asahi Shimbun newspaper.
The survey, taken last year and sponsored by the health ministry, came as the Japanese government mulls controversial plans to legalise casino gambling in certain special zones, with some saying it would boost the number of foreign tourists.
Researchers estimated that roughly 5.36 million people in Japan -- 4.8 percent of the adult population -- are likely pathological gamblers who cannot resist the impulse to wager, the Yomiuri Shimbun said.
The study said 8.7 percent of men and 1.8 percent of women fit the internationally-accepted definition of addicts, according to the Mainichi Shimbun.
The wide availability of pachinko parlours -- loud, colourful salons that offer rows of pinball-like games -- and other gambling establishments is believed to be contributing to the problem.
The ratio of compulsive gamblers in most nations "stands more or less around one percent of the adult population. So Japan's ratio is high," a member of the study group told reporters, according to the Nikkei newspaper.
The study questioned 7,000 Japanese adults nationwide, of whom 4,153 gave valid answers.
Around 4.21 million adults are believed to show signs of Internet addiction, the study found, a rate that had risen 50 percent in five years, the Nikkei said.
Researchers blamed the spread of smartphones and increasing quality of digital content for the rising number of IT addicts, who often prefer the Internet over other essential activities such as sleeping, the Nikkei said.
More than a million people were believed to be addicted to alcohol, compared with an estimated 830,000 people a decade ago, the Mainichi said.