Perfectionism is a consistent predictor of athlete burnout. Researchers have therefore sought to examine the psychological mechanisms that may explain this relationship. In the present study, guided by Smith’s cognitive-affective stress model, we extend existing research by examining whether perceived stress is one such explanatory factor. A sample of 256 adult athletes completed measures of perfectionism (perfectionistic strivings and perfectionistic concerns), perceived stress, and burnout. Correlational analyses indicated that perfectionistic concerns was positively related to burnout, while perfectionistic strivings was either negatively related or unrelated to burnout. Tests of bias-corrected bootstrapped indirect effects showed that perceived stress mediated the positive relationship between perfectionistic concerns and burnout. This finding was evident when examining total burnout and all three burnout symptoms. It appears that athletes high in perfectionistic concerns are likely to experience heightened levels of stress in sport which may in turn render them more vulnerable to burnout.