Anthropogenic activities and rapid urbanisation strongly influence natural ecosystems and their biodiversity. Natural areas on the border of expanding cities are particularly affected by anthropogenic pressures, possibly leading to a decrease or local extinction of animal species. The capital of Mongolia, Ulaanbaatar, is now hosting half of the entire country’s population and is rapidly expanding, impacting one of the oldest protected areas in the world, the adjacent Bogd Khan Mountain. Considering small mammals' key role in the ecosystems, and the scarce knowledge of Mongolian rodent ecology, we investigated the ground-dwelling small mammals on Bogd Khan Mountain, and assessed species assemblage and occurrence, essential for planning future conservation actions. We live-trapped rodents in two valleys (4 sites) between May and July 2023. We recorded five ground-dwelling rodents: Apodemus peninsulae, Craseomys rufocanus, Clethrionomys rutilus, Cricetulus barabensis, and Eutamias sibiricus. Historical records, however, showed a much higher species richness than the one recorded in this study. We discussed our findings in light of species ecology and potential threats to these populations. Our findings highlight the gaps in the understanding of small mammal ecology in Mongolia, emphasizing the need of further studies to ensure the conservation and protection of Bogd Khan Mountain and its wildlife.