A regent, from the Latin regens "who reigns" is anyone who acts as head of state, especially if not the monarch (who has higher titles). Thus, the common use is for an acting deputy governor. In a monarchy, a regent usually rules due to the actual monarch's absence, incapacity or minority, and may also be elected to rule during the sede vacante when the royal line has died out. This was the case in Finland and Hungary, where the royal line was considered extinct in the aftermath of World War I. In Iceland, the regent represented the King of Denmark as sovereign of Iceland until the country became a republic in 1944.

In San Marino, an ancient independent miniature republic surrounded within Italy, the "Captains Regent", or Capitani Reggenti, are two officials elected annually as joint heads of state and of government.

In the Polish-Lithuanian Commonwealth (1569-1795), kings were elective, which often led to a fairly long interregnum. In the interim, it was the Polish Roman Catholic Primate who served as the regent, termed the "interrex" (Latin: ruler "between kings," as in ancient Rome).