Burnout has been highlighted as an important issue, not only in occupational settings but also among athletes. Optimists appear to be more resistant to burnout, which might be partly explained by lower levels of stress. The purpose of this study was to investigate the relationship between optimism and burnout symptoms in 217 athletes (139 males and 78 females, aged 16 to 19 years), while also examining stress as a mediator in this relationship. The results showed that optimism had a significant negative relationship with both stress and burnout. Mediation analyses indicated that perceived stress fully mediated the links between optimism and two symptoms of burnout, emotional/physical exhaustion and sport devaluation, and partly mediated the link between optimism and a third symptom, reduced sense of accomplishment. The findings indicate that individual factors, such as optimism, may play an important role in the development of burnout by virtue of their association with stress. Future research should, therefore, investigate the longitudinal effects of optimism on stress and burnout.