The incidence of skin cancer in our society is growing at an alarming rate due to overexposure to solar UV radiation in recreational and occupational settings. The aim of this study was to evaluate sun exposure and protection attitudes, behaviors, and knowledge among mountain ultramarathon runners and to assess risk factors for sunburn in this population. Cross-sectional survey of runners who participated in the Gran Trail Aneto-Posets race in Aragon, Spain. Using a validated questionnaire, we collected data on sociodemographic characteristics, running experience, sunburn in the previous summer, and sun exposure and protection behaviors. We calculated descriptive statistics and performed bivariate and multivariate analyses of associations using history of sunburn as the primary outcome. Statistical significance was set at a P level of less than .05. We surveyed 657 runners (72.1% men) with a mean age of 39.71 years; 45.1% reported sunburn in the past year. The most common protective measures used were sunglasses (74.7%), sunscreen (sun protection factor ≥ 15) (61.9%), a hat (52.2%), and other protective clothing (7.4%). Risk factors for sunburn were younger age, low Fitzpatrick skin type (I and II), running for 3 or more hours a day, and staying in the shade as a protective measure. By contrast, protective factors were use of sunscreen and seeking shade rather than sun at midday (P < .001). Sunburn is common among long-distance mountain runners, despite what appears to be adequate sun protection knowledge and behaviors. Targeted strategies are needed to improve sun protection behaviors among mountain runners.