IAU may refer to:

• International Astronomical Union
• International American University
• International American University College of Medicine
• International Association of Universities
• International Association of Ultrarunners (ultramarathoners)
• Islamic American University
• Islamic Azad University
• Italian Actors Union, former name of the Guild of Italian American Actors, United States
• 5000 IAU, an asteroid
• Is Anyone Up?

The International Astronomical Union (IAU; , UAI) is a collection of professional astronomers, at the PhD level and beyond, active in professional research and education in astronomy. It acts as the internationally recognized authority for assigning designations to celestial bodies (stars, planets, asteroids, etc.) and any surface features on them.