A fluorescent “off-on” gamma (γ) dosimeter based on an imino-BODIPY dye is developed. The dosimeter is highly sensitive with the lowest limit of detection (LOD, 0.001 Gy) reported so far for gamma dosimeters based on any analytical method. The dosimeter also shows colorimetric changes under γ-exposure, which is useful for quick detection of the γ-doses. A detailed mechanistic study was done to explain these colorimetric and fluorometric changes. Thus, the dosimeter will be useful for measuring accurate gamma-doses in medical applications due to its high sensitivity. Interestingly, the dynamic range of the dosimeter can be tuned by changing the dye concentration. The dosimeter with 57.0 μM dye is capable of colorimetric and fluorometric measurement of 25 Gy γ-dose, which is useful for quick and accurate measurement of absorbed γ-doses during blood irradiation.