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Temperate, montane amphibians are experiencing rapid environmental change. Better mechanistic understanding of the response of montane amphibians to accelerating changes in air and water temperatures can provide a basis for robust predictions of the vulnerability of species to future climate change. Thermal performance curves allow for quantitative predictions of responses to temperatures beyond which locomotor physiology is compromised. We designed a field-based thermal performance assay to evaluate the frequency that adult Cascades Frogs (Rana cascadae) are exposed to harmful environmental temperatures. We fit a suite of generalized additive models in an information theoretic framework to estimate thermal performance curves and found Rana cascadae optimal performance temperatures (T-opt) vary as a function of mass. The T-opt for the median mass of individuals in our study (17.9 g) was 20.7 degrees C and critical thermal maximum (CTmax) was 34.0 degrees C. We calculated the number of days during the growing season (1 July to 30 September) that the temperature exceeded the upper bound of the 80% maximal performance breadth (T-br80), a conservative performance metric beyond which activity drops substantially, and CTmax for the median, 25th, and 75th percentile masses in our study using contemporary weather data from 1990 to 2015. Rana cascadae did not experience temperatures exceeding CTmax, but Tbr80 was exceeded 3-13 d per summer, depending on frog size. Our results emphasize that larger individuals may be more susceptible to extreme warm temperatures. Thermal performance studies not accounting for individual variation should be interpreted cautiously.