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Abstract Temperature extremes often limit animal distributions. Whereas some poikilotherms (e.g., winged insects) can escape local thermal extremes, many less vagile organisms (e.g., insect larvae and arthropods with limited dispersal ability) are at the mercy of local microenvironmental conditions. Here, we quantified the thermal tolerance of an abundant, endemic, Nearctic millipede (Euryurus leachii), and explored the effects of seasonality, mass, and sex on its critical thermal maxima (CTmax). We also measured the thermal microenvironments of dead wood representing different decay classes. Overall, the mean CTmax for this species was ca. 40.5°C. Mass and sex had no effect on millipede CTmax. However, the mean CTmax for millipedes collected in the fall was 0.6°C higher than for individuals collected in the spring. An exposed dry log representing one common microhabitat for E. leachii readily warmed to temperatures exceeding its CTmax. The results suggest that CTmax is a seasonally plastic trait in E. leachii and that microclimatic conditions potentially limit the local distribution of this species. With habitat fragmentation and climate change contributing to warmer temperatures in forested systems, understanding the responses of detritivores like E. leachii can help predict potential shifts in community composition and ecosystem processes.