Marriage and koseki

My fiance is Japanese and we would like to get married soon in Japan. I am half-Japanese and seem to have dual citizenship since birth. I was born abroad and in my country I was never informed about this. I would like to know how to register my marriage - as a Japanese in Japan to change my family register? But what will happen with my citizenship? Has someone had the same experience and know how to proceed. Thank you!

Congratulations on your engagement.

If you are 21 (or 19 depending on your birthday) years old or younger, there is no problem whatsoever in keeping both a Japanese nationality and another one.

If you have reached the age of 22, the Japanese government highly encourages you to abandon one of your dual nationalities, but dual nationality is also tolerated. So, technically speaking, it is possible to keep both your nationalities.

If two Japanese people need to register their marriage outside Japan, they should contact their local Japanese embassy or consulate.

This is all I know. I hope someone with experience or better and updated knowledge can help you. The following links are just for reference.

https://www.moj.go.jp/MINJI/minji78.html
https://style.nikkei.com/article/DGXMZO36579310X11C18A0EAC000/
https://djjon.es/?p=841
https://www.moj.go.jp/MINJI/minji15.html

If you didn't know that you still had the Japanese nationality, then probably you have no Japanese passport, i.e. you have been living abroad as a non-Japanese national with another country passport.

I encourage you to take this opportunity to do the formalities regarding the loss of your Japanese citizenship. You will marry as a foreign national, which is not that complex.

If you were born between a Japanese national and a non-Japanese national in a foreign country, and you obtained the citizenship of that country by birth, in addition to reporting your birth, your Japanese parent would have had to make another report to retain or reserve the Japanese citizenship for you within a short period of time, otherwise you would have lost the Japanese citizenship.

If your Japanese parent had reported your birth to the Japanese authorities, youfd have been entered into your parentfs gkosekih (family register) as the child, based on which you might be able to apply for the resident status gspouse/child of Japanese nationalh to come and stay in Japan.

Is there any way to check with your parent about the circumstances?