IOI may refer to:
- Idiopathic orbital inflammatory disease
- International Olympiad in Informatics
- Interonset interval
- Indication of interest, used in financial trading
- Interest on Investment, a synonym for Return on Investment (ROI)
- Island of Ireland, used to refer to the entire geographical entity encompassing both the Republic of Ireland (ROI) and Northern Ireland (NI).
- IO Interactive, a Danish video game developer
- International Ombudsman Institute
- IOI Group, a Malaysian conglomerate
- I.O.I, a South Korean girl group formed from the television show Produce 101
- Innovative Online Industries (IOI), the fictional corporation that employs most of the antagonists of the 2011 novel Ready Player One and its 2018 film adaptation