In a Quantum Cascade Detector, photocurrent is generated by the absorption of infrared and terahertz radiation in the quantum-well-based modules arranged in series. Consequently, the current responsivity is by construction inversely proportional to the number of cascading modules. Upon absorption of a photon, the electron travels through only a single period of the detector, with a mean free path corresponding to the period length. Therefore, the shot noise power density is expected to decrease by the same factor under sufficiently high illumination, reflecting the same inverse relationship with the number of cascading modules. This phenomenon leads to sub-Poissonian noise characteristics. We experimentally observe this effect in a 90-period Quantum Cascade Detector operating at 4.5 μm, confirming a reduction in the shot noise contribution by the anticipated Fano factor of 1/90. This measurement underscores the suitability of these detectors for coherent detection scenarios, particularly where shot noise dominates.