Common-law marriage, also known as non-ceremonial marriage, sui iuris marriage, informal marriage, or marriage by habit and repute, is a legal framework where a couple may be considered married without having formally registered their relation as a civil or religious marriage.

The original concept of a "common-law marriage" is one considered valid by both partners but not formally recorded with a state or religious registry, nor celebrated in a formal civil or religious service. In effect, the act of the couple representing themselves to others as being married and organizing their relation as if they were married, means they are married.

The term common-law marriage (or similar) has wider informal use, often to denote relations that are not legally recognized as marriages. It is often used colloquially or by the media to refer to cohabiting couples, regardless of any legal rights or religious implications involved. This can create confusion in regard to the term and to the legal rights of unmarried partners (in addition to the actual status of the couple referred to).