Neuromuscular and aerobic capacity can be reduced in people with intellectual disabilities (ID). Previous studies suggest these individuals might be particularly susceptible to losing basic functions because of poor physical fitness. The aim of this study is to describe the physical fitness profile of adult athletes with ID and identify whether there are differences in the physical performance between the most physically active individuals and less active individuals. A cross-sectional observational study was developed involving 266 athletes with mild ID (187 males and 79 females), recruited from the Spanish Special Olympics Games. A questionnaire was used to evaluate the health status of participants and their frequency of physical activity practice. A battery of 13 fitness tests was applied to assess flexibility, strength/endurance, balance and cardiovascular capabilities. Of the total participants, 44.3% were classified as sportspersons and the remainder as non sportspersons, taking in consideration the frequency of physical activity. Regarding the scores, a significant difference was found in degrees of flexibility between genders, higher for females for one test but higher for males in the other three. A significant difference was not encountered between other variables of physical fitness, although the men's scores were higher in strength/endurance and balance. When the scores of the sportspersons and non sportspersons groups were compared, no significant difference was found between the two, with the exception of one test for flexibility. Differences among groups and gender were not statistically significant in most of the tests. The findings in this study illustrate an unclear and inconclusive relation between the scores and the declared level of physical activity, maybe due to the context in which participants for the study were selected.