A koseki (Japanese: 戸籍) or family register is a Japanese family registry. Japanese law requires all Japanese households to make notifications of their vital records (such as births, adoptions, deaths, marriages and divorces) to their local municipal authority. Domestic aspects such as marriages, divorces, acknowledgements of paternity of non-marital children, and adoptions, are only legitimized if they are recorded on the koseki. Births and deaths become legally effective as they happen, but such events must be filed by family members or other persons as allowed by law.

Koseki registration is required for all Japanese citizens, and possessing one is definitive proof of Japanese nationality, unless a renunciation application has been filed and recorded on the koseki for reference. In addition to koseki registration, Japanese citizens are also required to file a notification of residence, which is then recorded in the jūminhyō (住民票). Foreign residents, who cannot have a koseki, are only required to file a notification of residence and have a jūminhyō.

Introduced in the 6th century, the original population census in Japan was called the kōgo no nenjaku (庚午年籍) or the kōin no nenjaku (庚寅年籍). This census was introduced under the ritsuryō system of governance. During the Shogunate, there were four major forms of population registration: the ninbetsuchō (人別帳) (Registry of Human Categories), the shūmon jinbetsu aratamechō (宗門人別改帳) (Religious Inquisition Registry) also called the shūmon aratamechō, the gonin gumichō (五人組帳) (Five Household Registry) and the kakochō (過去帳) (Death Registry). The shūmon jinbetsu aratamechō was created around 1670 and lasted almost 200 years. It combined social and religious registration, and data was renewed annually. Several categories of outcasts were not registered at all under this system, or were registered in specific registers, for instance the burakumin. 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. Although all previous social categories were abolished and almost all Japanese people were recorded as heimin (commoners), some minorities became labelled as "new commoner" or "original eta" (shinheimin or motoeta), and discrimination went on. Problems also happened at the edge of the national territory, for instance in the Ogasawara Islands.

During the course of the Empire of Japan, a number of reforms were carried out after 1910 to eliminate double standards in the koseki system. In general, though, residents of the Empire's colonies held external registries (gaichi koseki) (based on the preexisting Hoju) and Japanese held domestic registries (naichi koseki).

After the full revision of the Family Register Act in 1947 (enforced the following year), the household, known as "ie" were redefined to a narrower scope (married couples and their unmarried children), thus limiting the maximum number of generations under the same koseki to two generations.

In 2003, the "GID Law" was enacted, enabling people with "gender identity disorder" (GID) or gender dysphoria to change their gender on their koseki provided they meet certain conditions. Persons diagnosed with GID must seek an official diagnosis with letters of support from two independent psychiatrists to change their koseki gender. A person with functional reproductive glands or a married person cannot change their koseki gender.

There are two main types of certified copies of koseki: the Comprehensive Copy of Koseki (戸籍謄本, koseki tōhon) and Selected Copy of Koseki (戸籍抄本, koseki shōhon). The comprehensive koseki is a record of all family members, while the selected koseki is the information for only one individual family member.

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.