Normally in Japan, married couples are required to share the same last name. This requirement is based on the need to centralize the family in one family registry under one single name. When a baby is born to a couple with the same last name, the child’s surname in Japan will naturally be the name of its parents.
However, for Japanese nationals married to foreign nationals overseas, it is not necessary for both spouses to share the same last name. In these circumstances, when a baby is born between spouses with different last names, child’s surname in Japan will automatically be the last name of the Japanese parent in official Japanese government records (koseki tohon). This is because the baby will automatically be put into the Japanese parent’s family register and therefore it must share the same last name. In order to change the baby’s last name it is necessary to petition the family court in Japan to create a new family register for the child in Japan under the new last name.
It is important to note that, if a child’s name in Japan is different than their name registered abroad, this could cause inconveniences for international travel if the child has two different passports with different names on each.