A green low-temperature method for the synthesis of nanostructured KPbBr₃ perovskite is presented. The obtained nanocrystals are integrated into a hydroxyethyl cellulose (HEC) matrix, at RH = 95% the photosignal decreases by 85%, the activation energy of moisture-induced PL quenching is 24 ± 3 kJ mol⁻¹. The prototype of the sensor with interdigital electrodes showed a linear correlation of the photocurrent with bulk soil moisture of 8–25% (R² = 0.96) and a response time of 3.5 min with a power consumption of ≤ 12 mW, which allows the device to be powered by small solar cells. Thus, the KPbBr₃-HEC nanocomposite is a promising component of autonomous agrosensors for monitoring soil moisture and quality in arid regions.