In the currency system of early modern Japan, concurrent with gold coins and silver ingots issued under monopoly by the Tokugawa shogunal government from the beginning of its rule, in 1636 a new standard for copper coins was introduced with the Kan'ei tsūhō 寛永通宝, and subsequently a three-currency system spread throughout the country. Prior to that, no central Japanese authority had issued its own copper currency since the ancient imperial court discontinued minting coins bearing its own era-names in the tenth century. In the late sixteenth and early seventeenth century, imitation Chinese Song copper coins minted domestically, known as kyōsen 京銭, played an indispensable role in interregional payments, while separate regional coinages circulated in many of the feudal domains. Imitation Song coins from Japan were also in such strong demand in Southeast Asia that merchants Japanese as well as foreign made large profits by exporting them. The decision to mint the Kan'ei tsūhō arose in part from a strategy to unify the currency domestically, and also to prevent further production and export of the kyōsen at a time when the Tokugawa government was seeking to limit and control foreign trade. This study explores the context in which the Kan'ei tsūhō was introduced, drawing on both documentary and archaeological evidence, to establish a more comprehensive picture of the new coinage than has previously been available. It will be demonstrated that currency unification within the Japanese archipelago represented a breakaway from the East Asian currency sphere, which was dominated by purer silver ingots and Song coins, both originals and later imitations. As a result of the diffusion of the Kan'ei tsūhō, coins circulating domestically tended to become increasingly homogenized nationwide, though in fact a greater degree of regionality and stratification remained than has previously been thought.