Hyrcanian forests are considered among the oldest forests in the world, and due to their temperate and humid climate and fertile soil, they have rich fauna and flora. Soil-dwelling arthropods play essential roles in forest ecosystem functioning, contributing to litter decomposition, nutrient cycling, and soil structure maintenance. This study assessed the diversity and vertical distribution of macrofauna and mesofauna in Semeskandeh Forest, a protected Hyrcanian Forest in northern Iran, across different soil depths (leaf litter, 0–3 cm, 3–6 cm) and seasons over one year. Over 10,000 individuals were collected, representing diverse taxonomic groups, including ants, millipedes, springtails, mites, pseudoscorpions, and diplurans. Population densities and species diversity were consistently highest in the litter layer and declined with increasing soil depth, with clear seasonal peaks observed during the study period. Ants dominated the macrofauna, while mites and springtails were the most diverse mesofaunal taxa. The study revealed clear seasonal and vertical patterns in the density and diversity of soil faunal communities in the Hyrcanian forest ecosystem. These findings provide baseline information for biodiversity conservation and future monitoring of soil fauna in Hyrcanian forests. Thereby, conserving litter quality, maintaining habitat integrity, and monitoring soil faunal diversity are critical for sustaining forest ecosystem functions in such a globally valuable and inherited region.