RI or Ri may refer to:

• RI, the postal abbreviation for Rhode Island, a US state
• Ri, a common Korean surname
• Ri (administrative division), an administrative unit in both North Korea and South Korea
• Ri (kana), a Romanization of the Japanese kana り and リ
• Ri, Orne, a commune of the Orne département, in France
• Rí, the ancient Gaelic word meaning King
• Raffles Institution, a school in Singapore
• Refugees International, an international humanitarian organization
• Rehabilitation International, an international disability rights organization
• Arterial resistivity index
• Rex Imperator or Regina Imperatrix, a suffix used by British monarchs as Emperors of India
• Ring Indicator, a signal in the RS232 serial communications standard
• Rotary International
• Royal Institution of Great Britain
• Royal Institute of Painters in Water Colours, a society in the Federation of British Artists
• Chicago, Rock Island and Pacific Railroad's reporting mark
• Mandala Airlines's IATA airline designator
• Rhombicosahedron's Bowers acronym
• Refractive index of an optical medium (in physics/optics) is a number that describes how light propagates through that medium
• Ri, an augmented second in Solfege
• Ri, a representation of the second Swara in Indian classical music
• Ri, a proposed gender-neutral pronoun for Gender reform in Esperanto
• Republic of Indonesia, or, in Indonesian language, "Republik Indonesia"
• Ri, a unit of measurement in Japan and Korea

Rí, or very commonly ríg (genitive), is an ancient Gaelic word meaning "king". It is used in historical texts referring to the Irish and Scottish kings, and those of similar rank. While the Modern Irish word is exactly the same, in modern Scottish Gaelic it is rìgh, apparently derived from the genitive. Cognates include Gaulish Rix, Latin rex/regis, Sanskrit raja, and German Reich.

A ri or village is an administrative unit in both North Korea and South Korea similar to the unit of Village.