A new ternary metal halide scintillator, potassium strontium iodide, activated with divalent europium (KSr2I5:Eu) has been discovered. This material has a monoclinic crystal structure with a density of 4.39 g/cm3. Differential scanning calorimetry indicates a congruent melting point of 470 °C and suggests that this compound has no solid–solid phase transitions. As is the case with most metal halides, the material is hygroscopic, and it has some internal radioactivity due to the presence of 40K. Single crystals of KSr2I5 doped with 4% Eu2+ were grown in evacuated quartz ampoules via the Bridgman technique. The X-ray excited emission spectrum consisted of a single peak at ~445 nm due to the 5d–4f transition in Eu2+. The measured light yield is ~94,000 photons/MeV with an energy resolution of 2.4% at 662 keV. The crystal has an excellent proportionality response over a wide range of energies from 14 keV to 1275 keV.