The latest seizure, with an estimated value of $73 million, more than doubles the previous record set in a 177kg haul at another port.
Japanese authorities have seized a record-breaking 400 kilograms of cocaine worth an estimated $73 million in a bust at a port in the city of Kobe, a customs spokesman said on Friday.
The discovery in the western city was made last month but came to light on Friday, with officials saying they could not provide additional details because the investigation was ongoing.
"This is record-breaking, in terms of volume" for a cocaine bust, the spokesman told AFP.
It far exceeds the previous record set just months earlier in August, when authorities seized 177 kilograms of cocaine at another port, in central Japan's Aichi prefecture.
It was unclear yet where the cocaine came from, and Kyodo News agency, citing unnamed investigative sources, said the drugs may have been sent to Kobe by mistake.
Japan has strict drug laws and users face severe public opprobrium if caught.
Drug use and possession by even moderately famous people, including television actors and little-known athletes, can make national headlines for days.
Experts warn however that casual drug use among young people is becoming a growing problem.
Drug sales are a major fin ancial pillar for Japanese organised crimes, including the infamous yakuza.