It said men who drank five or more cups a day might halve the risk of developing advanced prostate cancer compared with those who drank less than one cup a day.
"This does not mean that people who drink green tea are guaranteed to have reduced risk of advanced prostate cancer," said Norie Kurahashi, a scientist who took part in the study.
"We are just presenting our results. But the study does point to the hope that green tea reduces the risk of advanced prostate cancer."
Prostate cancer is much less common among Asian men than Western men, and that may be partly due to the effects of the high consumption of green tea in Asia, the study said.
But it said further studies are needed to confirm the preventive effects of green tea on prostate cancer, including well-designed clinical trials.
The study, published in the American Journal of Epidemiology, compiled data from 50,000 men aged 40-69 over a period of up to 14 years from 1990.
British charity Cancer Research UK says on its Web site that a study of almost 20,000 Japanese men published in the British Journal of Cancer in 2006 found no relationship between green tea and prostate cancer.
Reporting by Chisa Fujioka, Editing by Michael Watson)