Temperature is a critical factor in the distribution of stream fishes. From laboratory studies of thermal tolerance, fish ecologists can assess whether species distributions are constrained by tolerable thermal habitat availability. The objective of this study was to use lethal thermal maxima (LTM) methodology to assess the upper thermal tolerance for mountain sucker Catostomus platyrhynchus, a species of greatest conservation need in the state of South Dakota. Adult fish were captured from wild populations in the Black Hills of South Dakota and acclimated to 20, 22.5, and 25 °C. Four endpoints (3 sublethal, 1 lethal) were recorded, with death being the most precise (lowest SE, easily discernible). The LTM for mountain sucker was 34.0 °C at 25 °C acclimation, 33.2 °C at 22.5 °C acclimation, and 32.9 °C at 20 °C acclimation. Compared to co-occurring species in the Black Hills, the LTM of mountain sucker is higher than that of salmonids but lower than that of 3 cypriniforms. Mountain sucker LTM is intermediate compared to other species in the family Catostomidae. These results suggest that the mountain sucker is not currently limited by water temperatures in the Black Hills but may be affected by stream warming as a result of climate change.