R. or r. may refer to: * ''Reign'', the period of time during which an Emperor, king, queen, etc., is ruler * ''Rex (title), Rex'', abbreviated as R., the Latin word meaning King * ''Regina'', abbreviated as R., the Latin word meaning Queen regnant, Queen * or , 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. (R. Kelly album), R.'', an album by American singer R. Kelly

See also