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A koseki (戸籍) is a family registry. Japanese law requires all households (ie) to report births, deaths, marriages, divorces, and criminal convictions to their local authority, which compiles the information into a detailed family tree that encompasses everyone within their jurisdiction. If such events are not recorded in the koseki, they are not officially acknowledged by the Japanese government.

While similar systems have been employed in Japan since ancient times, 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. Records were originally kept in lengthy paper volumes, but were translated to digital format in 2002 and are now entirely computerized.

The koseki is traditionally held by the eldest male descendant of the lineage, the honke. During times of veneration of the family's ancestors, Higan (spring) and Obon (fall), the other family members (bunke) may visit the home of the honke to venerate the family's ancestors.

The koseki simultaneously fills the function of birth certificates, death certificates, marriage licenses, and the census in other countries. It is also based on family rather than each individual. Information provided in koseki is detailed as well as sensitive and makes discrimination possible against such groups as burakumin or illegitimate children and lone unwed mothers, for example. As burakumin liberation movement gained strength in postwar Japan some changes were made to family registry. In 1970 some details of one’s birth address were deleted from the family registry. In 1974 a notice that prohibited employers to ask prospective employees to show family registry was released by the Ministry of Health and Welfare. In 1975 one's lineage name was deleted and in 1976 access to family registry was restricted. Nowadays the Family Registry records are usually held in strict privacy by the government, and are usually shown only to family members or the police. Anyone who is listed on a koseki, even if their name has been crossed off by reason of divorce and even if they are not a Japanese citizen, is legally able to get a copy of that koseki. They can get one in person in Japan, or by mail outside of Japan. Lawyers can also get copies of any koseki if a person listed is involved in legal proceedings.

A typical koseki has one page for the household's parents and their first two children: additional children are recorded on additional pages. Any changes to this information have to be sealed by an official registrar.

Note that the koseki system is different from the jūminhyō, or Residency Registration system, with which is it sometimes confused.

A similar household registration system exists within the public administration structures of China (hukou), Vietnam (ho khau), and both North and South Korea (Hoju). In South Korea the (Hoju) system was repealed in 2005

• Practical information on obtaining and using a koseki for Family Law related purposes in Japan
• Information on Japan's Jûminhyô, or Residency Registration system