R.

R. or r. may refer to:


 * Reign, the period of time during which an Emperor, king, queen, etc., is ruler.
 * Rex, abbreviated as R., the Latin word meaning King
 * Regina, abbreviated as R., the Latin word meaning Queen
 * Regnavit or rexit, abbreviated as r., used in historiography to designate the ruling period of a person in dynastic power, to distinguish from his or her lifespan (e.g. "Charles V (r. 1519–1556)")
 * Abbreviation R., meaning Rex or Regina. Commonly used for criminal prosecution in Commonwealth realms where offences are brought in the name of the monarch (e.g. "R. v Defendant")
 * R., an album by American singer R. Kelly