Iao may refer to:

• IAO (album), an album by avant jazz musician John Zorn
• Iao (Gnosticism), an archon corresponding to Jupiter
• Iao Theater, a theater in Wailuku, Hawaii
• Iao Valley, a tourist attraction in Maui, Hawaii
• Sayak Airport (IATA code: IAO), also known as Siargao Airport, in Surigao del Norte, Philippines
• ιαω, an early Greek form of the tetragrammaton, or name of God

IAO may stand for:

• Incorporated Association Of Organists
• Indian Astronomical Observatory, home of the highest telescope in the world
• Information Awareness Office, a branch of the U.S. Defense Advanced Research Projects Agency (DARPA)
• Interests, Activities, and Opinions - IAO or psychographic variables
• International Accreditation Organization (Houston, TX)
• International Academy of Osteopathy, an international school offering academic training in osteopathy
• International Astronomy Olympiad, an astronomy competition for high-school students
• Interparliamentary Assembly on Orthodoxy, an organization of parliamentarians of the Eastern Orthodox Church
• Institut d'Asie Orientale (Lyons Institute of East Asian Studies), a centre for research in human and social sciences at Université Lumière-Lyon 2 and Institut d'études politiques de Lyon
• Institute for Art and Olfaction, a non-profit organization
• Istituto agronomico per l'oltremare (Overseas Agronomic Institute), a technical and scientific body of the Italian Ministry of Foreign Affairs