An investigation of illegal and unreported fishing in source countries, including product mixing during movement through intermediary countries and reprocessing in the product supply chain, indicates that 24-36% of 2.15 million tonnes of wild-seafood imports to Japan in 2015 contained material of illegal and unreported origin, valued at $1.6 to $2.4 billion, out of a total import of US$ 13 billion. Estimates, with appropriate uncertainties, were made using information from over 312 sources including 121 structured interviews with anonymous correspondents relating to 27 product import chains and in data poor fisheries. A supply chain case study of crab imported from Russia illustrates the intricacies of trade in illegal seafood products in this region. Weakly framed import regulations and outdated fisheries policies appear to be driving an inadvertent trade of illegal and unreported seafood products into the Japanese market. A multilateral approach could extend the strict requirements for food safety to the country of origin and provide more complete supply chain traceability for all commercial seafood species imported by Japan.