An unregistered marriage (nikah sirri) takes place in the presence of a guardian, two witnesses, and ijab qabul (Islamic marriage contract). Unregistered marriage is a legal marriage, but it is not recorded in state documents. It means the country does not recognize marriage, even though it is religiously legal. Unregistered marriage is frequently used as an alternative for a small number of people who want to marry but are hampered by time and money-consuming administration, such as permission to polygamy. Because polygamy permits have cumulative requirements, unregistered polygamy marriage allows perpetrators to remarry easily. This will be a problem if a child is born from such a marriage, as the child will require legal certainty regarding his or her civil rights. Based on this reality, perpetrators of unregistered marriage must apply to the Religious Court for ratification of unregistered polygamy marriages. This study used an empirical juridical method, in which the author compared how marriage law is regulated in Indonesia to existing field facts. Based on the results of the research, this incident was legal smuggling (illegal polygamy), which the country must strictly regulate because applications for unregistered polygamy marriage certificates still exist in the Religious Courts. This indicates that unregistered marriage for polygamy is still prevalent in society.