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    R may refer to:

    • R, denoting a member of the Republican Party in American politics
    • R, denoting The Crown, Rex (King) or Regina (Queen) in Commonwealth nations
    • R. (album) (1998) by R. Kelly
    • R (cable operator), a telecommunications company
    • A film rating in the Motion Picture Association of America film rating system
    • Rated R (Queens of the Stone Age album)
    • Rolled R, the alveolar trill of Spanish, Italian, & Arabic languages
    • The three Rs of Reading, wRiting and aRithmetic
    • In The World Is Not Enough, a character played by John Cleese
    • R·Type, an arcade video game
    • ®, the registered trademark symbol
    • The South African rand currency
    • Ryder's NYSE ticker symbol
    • R (New York City Subway service)
    • R, the military designation for a reconnaissance aircraft.
    • Volkswagen Golf R
    • The spectral resolution in astronomy
    • The symbol given to the molar gas constant in chemistry
    • A stellar classification for orangish or K carbon stars
    • R (cross section ratio), the ratio of hadronic to muonic cross sections
    • Röntgen, a unit of measurement for ionizing radiation such as X-ray and gamma rays
    • The Rydberg constant, a physical constant relating to energy levels of electrons within atoms
    • R, a label that denotes one of two chiral center configurations in the R/S system
    • A side chain (an unspecified group of atoms) in a chemical or structural formula
    • Arginine, an amino acid
    • Relatedness, in biology
    • r, representing population growth rate in the r/K selection theory of ecology
    • Haplogroup R (mtDNA), a human mitochondrial DNA (mtDNA) haplogroup
    • Haplogroup R (Y-DNA), a Y-chromosomal DNA (Y-DNA) haplogroup
    • ATC code R Respiratory system, a section of the Anatomical Therapeutic Chemical Classification System
    • °R, the symbol for the thermodynamic temperature scale Rankine
    • R-value (insulation), a non-SI measure of thermal resistance used in housing insulation
    • R-value (soils), property of soils used in pavement design
    • An electronic resistor, e.g. in circuit diagrams
    • The R-# refrigerant numbering system
    • In fracture mechanics, the ratio between the minimum and maximum stresses
    • R (programming language), an environment for statistical computing and graphics
    • R (complexity), the set of all recursive languages
    • R, RequestBot, a service in QuakeNet's IRC services
    • IBM System R, an IBM database system