Background  It is well established that there is a relationship between physical inactivity and increased risk for diseases of lifestyle. Persons with Down syndrome (DS) are especially at risk because of physical and health impairments, as well as perceived and real barriers to participation in exercise. The purpose of the study was to establish the functional fitness capacity and predictors of performance of DS adults.Methods  Data were collected at various intellectual disability centres and private homes in seven provinces of South Africa. Three hundred and seventy‐one DS individuals (199 men and 172 women) from 18 to 66 years were tested for balance, flexibility, coordination, muscular strength and endurance, aerobic capacity and functional ability. Data were categorised according to gender and age groups (18–25, 26–35, 36–45, and >45 years). Multiple regression analysis was performed to determine the relationship between the functional task and physical test items.Results  Down syndrome men performed significantly better on all but two tests compared with the women (P < 0.05). DS women performed better on the sit‐and‐reach flexibility item and the chair stand test; however, differences were not statistically significant from the men. Significant differences across age groups were observed for nine of the 13 functional fitness tests (P < 0.05). Muscular strength items, especially leg strength, significantly predicted functional performance in DS men and women. Aerobic capacity only predicted functional performance in DS men and sit‐and‐reach flexibility and dynamic balance only in DS women.Conclusions  Findings of this study provide important information on the functional capacity of DS adults and show which physical attributes contribute to functional performance. Consequently appropriate training programmes can be tailored for this population whom is known to have poor functional fitness.