Background: The physical activity level of older adults with intellectual disabilities (ID) is extremely low, and their fitness levels are far beneath accepted norms for older people with normal intelligence and comparable with frail older people. A physical activity programme, including an education programme, was developed for older adults with ID using behaviour change techniques. The programme aimed at improving or maintaining adequate levels of physical activity (primary outcome measure) and motor fitness, cardio respiratory fitness, morphologic and metabolic fitness, activities of daily living, cognitive functioning and depressive symptoms (secondary outcome measures). Method: The programme's efficacy was evaluated in a cluster-randomised clinical trial among people aged 43 years and over with mild-moderate levels of ID. Five day-activity centres were randomised to the participation group. In these centres, 81 older adults participated in groups of 8 to 10 in the programme, three times a week during 8 months. The programme was executed by physical activity instructors and staff of day-activity centres. Five other day-activity centres were randomised to the control group; 70 older adults in these centres received care as usual. The generalised linear model with mixed effects was used to test the programme's effectiveness. Results: Significant effects were found on physical activity, muscle strength, systolic and diastolic blood pressure, serum cholesterol level and cognitive functioning, in favour of the programme's participants. No significant improvements were found on balance, serum glucose, weight, waist circumference, walking speed, mobility, depression or instrumental activities of daily living. Conclusions: The physical activity and fitness programme has established small but significant effects in this sample, but generalising the findings to other settings is difficult due to significant participant dropout. Implementation of evidence-based physical activity programmes among older adults with ID is recommended. Further research is needed to investigate the effectiveness of physical activity on daily life functioning and the development on chronic diseases in the long run.