IOI can refer to:
- International Olympiad in Informatics
- Internet Open Index, a free web search index
- Interonset interval
- Indicator of interest, gauge in seduction literature
- Indication of Interest, used in financial trading
- IO Interactive
- International Ombudsman Institute
- IOI Group, IOI Corporation, industrial gas manufacturing in Malaysia, property development (Marriott Putrajaya Hotel and Palm Garden Hotel in Putrajaya), oil palm plantations
- Internal Object Identifier, used in ControlNet Messaging (industrial network protocol for industrial automation applications)