| IAA | International Academy of Astronautics |
| IAF | International Astronautical Federation |
| IAG | International Association of Geodesy |
| IAGA | International Association of Geomagnetism and Aeronomy |
| IAGC | International Association of Geochemistry and Cosmochemistry |
| IAMAP | International Association of Meteorology and Atmospheric Physics |
| IARU | International Amateur Radio Union |
| IATA | International Air Transport Agency |
| IAU | International Astronomical Union |
| IBCN | Integrated Broadband Communication |
| IBS | INTELSAT Business Service |
| IBW | Instantaneous Bandwidth |
| ICAO | International Civil Aviation Organization |
| ICAS | International Council of the Aeronatical Sciences |
| ICO | Intermediate Circular Orbit |
| ICSTI | International Council for Scientific and Technical Information |
| ICSU | International Council for Scientific Unions |
| IDR | Intermediate Data Rate |
| IEC | International Electrotechnical Commission |
| IEE | Institution of Electrical Engineers |
| IEEE | Institution of Electrical and Electronics Engineers |
| IF | Intermediate Frequency |
| IFF | Identification Friend or Foe |
| IFL | International Frequency List |
| IFRB | International Frequency Registration Board (ITU-R) |
| IFRC | International Frequency Registration Circular (ITU-R) |
| IIASA | International Institute of Applied Systems Analysis |
| IISL | International Institute of Space Law |
| ILS | Instrument Landing System |
| IMASS | Intelligent Multiple Access Spectrum Sharing |
| IMO | International Maritime Organization |
| IMT | International Mobile Telecommunications |
| IMT-2000 | International Mobile Telecommunication System |
| INAF | National Institute for Astrophysics (I) |
| INMARSAT | INternational MAritime SATellite organization |
| INPE | Instituto Nacional de Psequisas Espaciais |
| INSU | Institut National des Sciences de l’Univers (F) |
| INTA | Instituto Nacional de Tècnica Aerospacial |
| INTELSAT | International Telecommunications Satellite Organization |
| ION | The Institute of Navigation (US) |
| IR | Infrared |
| IRAM | Institut de Radio Astronomie Millimétrique |
| IRG | Independent Regulatory Group (EC) |
| IRT | Institut für Rundfunktechnik (D) |
| ISAR | Inverse Synthetic-Aperture Radar |
| ISAS | Institute of Space and Astronautical Science |
| ISDN | Integrated Services Digital Network |
| ISL | Inter-Satellite Link |
| ISM | Industrial, Scientific, Medical |
| ISO | International Organization for Standardization |
| ISR | Incoherent Scatter Radar |
| ISRO | Indian Space Research Organization |
| ISS | Inter-Satellite Service |
| IT | Information Technology |
| ITA | Industrial Telecommunication Association (USA) |
| ITFS | Instructional Television Fixed Service |
| ITU | International Telecommunication Union |
| ITU-D | International Telecommunication Union – Telecommunication Development Sector |
| ITU-R | International Telecommunication Union – Radiocommunication Sector |
| ITU-T | International Telecommunication Union – Telecommunication Standardization Sector |
| ITWG | International TOVS Working Group |
| IUCAF | Scientific Committee on the Allocation of Frequencies for Radio Astronomy and Space Science (UNESCO) |
| IUGG | International Union for Geodesy and Geophysics |
| IVS | International VLBI Satellite |
| IWG | Intersessional working group (of the SFCG) |
| IWP | Interim Working Party (ITU-R) |
| IZMIRAN | Institute of Terrestrial Magnetism, Radio Research and the Ionosphere (Russia) |