In the Empire of Japan, naturalization was initially covered by the 1873 Intermarriage Proclamation, and later defined by the the 1899 Nationality Law.

The 1873 proclamation provided that both a female alien married to a Japanese husband and a male alien adopted as a muko-yōshi by a Japanese parent-in-law would automatically “gain the standing of being Japanese.” The muko-yōshi (adpoted son-in-law) is a traditional term for someone adopted as one’s son by marring his/her daughter, and it has been a popular from of non-biological family succession in Japan. In case the parent-in-law was not alive, a male alien could be naturalized as a nyūfu (incoming husband), the legal term for a husband married to a female chief of a household. There were 59 cases of naturalization between 1886 and 1897, including the acknowledgement of illegitimate children.

The 1899 law opened a gate for naturalization without marriage. Under the law, an alien who had lived in Japan over five years consecutively, was over 20 years old, was “of good conduct and behavior”, and had “property or ability to earn an independent livelihood” had a chance to be naturalized with the permission of the Home Minister. These conditions are followed by the present 1950 Nationality Law.

The most famous Westerner naturalized in the Empire would be Lafcadio Hearn. Born in the British-occupied Ionian Islands, he married Setsu Koizumi in 1891, and naturalized himself as Yakumo Koizumi in 1986. As her father was not alive, first Setsu established a branch household seperated from her brother’s, and then Hearn applied for naturalization as her nyūfu.

Typical naturalization happened when a Japanese student married a local woman while studying in the West, and accompanied her home after he competed the course. Clara Louise Zitelmann, the German wife of a dendrologist Hazama Matsuno, established the first kindergarten in Japan and taught Friedrich Fröbel’s educational theory at Tokyo Women's Normal School. Jessie Roberta “Rita” Cowan, the Scottish wife of a sake brewer Masataka Taketsuru, helped him produce the first Japanese whisky and found what is now known as Nikka Whisky. Mariko Nitobe, born as Mary Patterson Elkinton in Philadelphia, was the wife of Inazō Nitobe, the author of Bushido, and the founder of Japan Humane Society, which advocated the welfare of oxen and dogs in Japan. Shigenori Tōgō, the wartime Foreign Minister later convicted at the Tokyo Trial, had a Jewish wife named Eji, who was born as Editha Giesecke in Germany.

The naturalization of male aliens were relatively rare, but one notable example is Kairakutei Black I. Born as Henry James Black in Australia, he was accompanied by his father to Japan, and made a living by the rakugo storytelling in Tokyo. Despite being homosexual, he married Aka Ishii as her mother Mine’s muko-yōshi in 1893, and acquired Japanese nationality as Burakku Ishii on the same day, which allowed him to perform all around the country without restriction.