| JBO | Jodrell Bank Observatory |
| JCMT | James Clark Maxwell Telescope |
| JEM | Japanese Experiment Module |
| JEWM | Joint Expert Working Meeting |
| JIVE | Joint Institute for VLBI in Europe |
| JIWP | Joint Interim Working Party |
| JPL | Jet Propulsion Laboratory |
| JPOP | Japanese Polar Platform |
| JSS | Joint Surveillance System |
| JTIDS | Joint Tactical Information Distribution System |