Background and Objective: Forensic entomotoxicology examines how toxins affect the development of arthropods that feed on decaying bodies, which can influence post-mortem interval (PMI) calculations. This research focuses on the impact of ZnCoS nanoparticles (NPs) on the decomposition stages of rat carcasses and their effects on Dermestes maculatus; a species of forensic relevance. Materials and Methods: Thirty albino rats were assigned to control and treatment groups, receiving different doses of ZnCoS NPs. The decomposition was observed daily for a month, with arthropods collecting regularly. The development rates and structural changes in Dermestes maculatus were examined using light microscopy, scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Results: Exposure to ZnCoS NPs resulted in a 22–33 hour delay in the PMI for specific insect species. Structural damage, especially to the wings of Dermestes maculatus, was evident, showing signs of apoptosis. These findings indicate that ZnCoS NPs alter both insect growth and the rate of decomposition. Conclusion: ZnCoS NPs have a notable impact on decomposition and PMI estimation, underscoring the importance of further forensic investigation into nanoparticle toxicity. SEM and TEM proved an efficiency in conducting postmortem toxicological analyses.