Iao or IAO may refer to:
- International Astronomy Olympiad, an annual astronomy competition for high-school students
- Indian Astronomical Observatory, home of the highest telescope in the world
- Istituto agronomico per l'oltremare (Overseas Agronomic Institute), a technical and scientific body of the Italian Ministry of Foreign Affairs
- Information Awareness Office, a branch of the U.S. Defense Advanced Research Projects Agency
- IAO variables (for Interests, Activities, and Opinions), another name for psychographic variables* IAO (album), an album by avant jazz musician John Zorn
- Iao Valley, a popular tourist attraction on Maui, Hawaii
- Wattlebird
- ιαω, an early Greek form of the tetragrammaton
- Iao, an archon in Gnosticism