The ryô was the basic unit of currency in Japan during the Edo era, and is said to have originated in the province of Kai before being approved by Tokugawa Ieyasu as a national standard. It was equal to just over fifteen grams of gold, or one koku of rice (the amount of rice needed to feed a retainer for a year): using modern Japanese rice prices as a benchmark, the ryo was worth around $150.

Several coins were based on the ryo, including the ten-ryo oban, one-ryo koban, and quarter-ryo ichibukin. At first, these gold coins were primarily used in eastern Japan, where the economy was not very well-developed at the time: in the west, silver currency based around the momme (50 momme = 1 ryo) was far more common. By the late 1600's, gold and silver were being used simultaneously in transactions across Japan: the exchange rate rose to 60 momme to the ryo by the late 1700's.

Immediately following the Meiji Restoration of 1868, the new government printed paper money based on the ryo. In 1871, however, the ryo was officially replaced by the yen, sen, and rin: by 1874, its instruments had been removed from circulation.