A queen regnant (plural: queens regnant) is a female monarch, equivalent in rank and title to a king, who reigns suo jure (in her own right) over a realm known as a "kingdom"; as opposed to a queen consort, who is the wife of a reigning king; or a queen regent, who is the guardian of a child monarch and rules pro tempore in the child's stead, be it de jure in sharing power or de facto in ruling alone. She is sometimes called a woman king. A princess regnant is a female monarch who reigns suo jure over a "principality"; an empress regnant is a female monarch who reigns suo jure over an "empire".

A queen regnant possesses and exercises sovereign powers, whereas a queen consort or queen regent shares her spouse's and/or child's rank and titles but does not share the sovereignty of her spouse or child. The husband of a queen regnant traditionally does not share the queen regnant's rank, title, or sovereignty. However, the concept of a king consort or prince consort is not unheard of in both contemporary and classical periods.

A queen dowager or empress dowager is the widow of a king or emperor; a queen mother is a queen dowager who is also the mother of a reigning sovereign.