A koseki is a Japanese family registry. Japanese law requires all Japanese households (ie) to report births, acknowledgements of paternity, adoptions, disruptions of adoptions, deaths, marriages and divorces of Japanese citizens to their local authority, which compiles such records encompassing all Japanese citizens within their jurisdiction. Marriages, adoptions and acknowledgements of paternity become legally effective only when such events are recorded in the koseki. Births and deaths become legally effective as they happen, but such events must be filed by family members.

The modern koseki, encompassing all of Japan's citizenry, appeared in 1872, immediately following the Meiji Restoration. This was the first time in history that all Japanese people were required to have family names as well as given names.