IAA is a three letter initialism that may stand for:
- In science and medicine
- Indole-3-acetic acid (plant hormone)
- Aux/IAA repressors, a class of early auxin responsive transcription factors
- Insulin autoantibody, used in diagnosing Latent autoimmune diabetes
- Interrupted aortic arch
- Intravenous ascorbic acid therapy for cancer or sepsis
- International Association for Aerobiology
- International Association of Astacology, for the study of Crayfish
- In healthcare organisations
- Independent Ambulance Association representing private ambulance services in the United Kingdom
- In aviation
- Indianapolis Airport Authority
- Iran Aseman Airlines, call-sign IAA
- Irish Aviation Authority
- Israel Airports Authority
- In computing
- Interactive Achievement Awards
- In education
- Idyllwild Arts Academy, part of the Idyllwild Arts Foundation
- IMSA Alumni Association
- Intercollegiate Athletic Association (formerly), now the National Collegiate Athletic Association (NCAA)
- Interlochen Arts Academy, part of the Interlochen Center for the Arts
- International Affairs Association of the University of Pennsylvania
- In politics
- Information Affairs Authority of Bahrain
- In sport
- International Ammunition Association
- International Aquathlon Association
- Other associations
- Immigration Appellate Authority (UK)
- Indian Agricultural Association
- Indian Association of Alberta
- Institute for Alternative Agriculture
- Institute for Applied Autonomy
- Institute of Actuaries of Australia
- Instituto Antártico Argentino
- Instituto de Arqueología Amazónica
- Instituto de Astrofísica de Andalucía
- International Academy of Architecture
- International Academy of Astronautics
- International Actuarial Association
- International Advertising Association
- International Alpaca Association
- Investment Adviser Association
- Irrigation Association of Australia
- Islamic Army of Aden
- Israel Antiquities Authority
- Israel Israeli Astronomical Association
- Other uses
- Intelligence Authorization Act (1991 - U.S.)
- Internationale Automobil-Ausstellung, Frankfurt Motor Show