Polygamy is illegal in Japan, with a strict monogamy law that allows a person to be legally married to only one spouse at any given time. Historically, Japanese men were allowed to have multiple wives, limited to those who could afford it. The shift towards monogamy began during the Meiji period as part of broader legal reforms aimed at modernizing Japanese society. Today, marriage customs are rapidly evolving as attitudes towards relationships change, with a growing number of people choosing to remain single or have non-traditional relationships.