The RADiation–hard Electron Monitor (RADEM) is an instrument on board the ESA JUpiter ICy moons Explorer (JUICE) deep-space mission launched on April 14th, 2023. As a part of the Cosmic Vision program, RADEM on JUICE will spend over three years exploring the radiation environment of the Jovian system, including its icy moons Ganymede, Callisto, and Europa. The instrument serves as an on-board radiation monitor, providing nonstop information on particle fluxes and their energy spectra. In addition to being a platform subsystem relevant to spacecraft safety and health, RADEM obtains scientifically valuable data on the radiation environment and extends the particle detection range covered by the JUICE Particle Environment Package (PEP) instrument suite to much higher energies, and broadens the energy coverage in the tens to hundreds of MeV range for electrons and protons, compared to past missions. RADEM consists of three detector subunits: the Electron Detector Head, the Proton & Heavy Ion Detector Head, and the Directional Detector Head. Each of them is connected to a separate readout electronics with a dedicated front-end Application–Specific Integrated Circuit (ASIC) designed especially for the JUICE mission. RADEM measures electrons in the 0.3–40 MeV energy range, protons in the 5–250 MeV energy range, and heavy ions within the Linear Energy Transfer range from 0.1 to 10 MeV cm mg−1. The Directional Detector provides an angular coverage of incoming radiation up to about 35% of the sky. Being a platform device, the monitor operates and delivers data permanently. Therefore, RADEM measurements also cover the radiation environment of the interplanetary space during the mission cruise phase, including long-term studies of the environment between Venus and Mars as well as the detection of the Solar Energetic Particle events that propagate across different locations in the Solar System.