<p>Climate and land-use change are driving global biodiversity declines, impacting species’ distributions, behaviours, and physiology. The hazel dormouse (<i>Muscardinus avellanarius</i>) is highly sensitive to environmental change, yet little is known about how climate change and land-use change influence its body mass. Using data from a 30-year citizen science monitoring programme, we investigated spatiotemporal trends in dormouse body mass across Britain and assessed the roles of climate and land cover as potential drivers. Furthermore, we tested whether changes in body mass vary between sexes and across seasons due to differences in energetic requirements. We show that post-hibernation body mass (May-June) has declined over time, in line with the general observed trend of body size decreasing with rising ambient temperatures to support thermoregulation. In contrast, pre-hibernation body mass has increased over time, potentially indicating increased resource availability linked to increased summer rainfall. Post-hibernation body mass was higher in warmer and wetter sites, while pre-hibernation body mass was higher at cooler and wetter sites. Landscape analysis showed a positive correlation between dormice body mass and the presence of hedgerows 4–6&#xa0;m tall, while there was a negative effect of overgrown hedgerows. This study provides the first insights into how climate and landscape affect hazel dormouse body mass, offering crucial guidance for conservation strategies amidst ongoing environmental change. Differences in response to climate change across the year highlight the importance of accounting for seasonal variability when studying impacts of climate change on heterothermic animals.</p>