Background: Overweight and obesity are major health risk factors in people with intellectual disabilities. The aim of this study was to test the effectiveness of a multicomponent programme (physical activity, diet and motivation) for overweight and obesity in adults with intellectual disabilities. Material and Methods: A quasi-experimental design with repeated-measures and non-equivalent control group (n = 33, n = 31) was used. The programme was conducted over 17 weeks, with follow-up at 6 months in a sample of Spanish adults with a mild and moderate intellectual disability from a community occupational day centre, aged from 23 to 50, 40.6% of which were women. Results: A significant reduction in weight and diastolic blood pressure was obtained over time, and this reduction was maintained in the follow-up for weight. Reduction in heart rate was only marginally significant. Conclusions: The treatment was effective in reducing overweight and obesity, improving cardiovascular capacity and therefore the physical health of the participants.