Background: Overweight/obesity and poor physical fitness are two prevalent lifestyle-related problems in older adults with intellectual disabilities, which each require a different approach. To improve healthy ageing, we assessed whether fatness or fitness is more important for survival in older adults with intellectual disabilities. Methods: In the HA-ID study, we measured obesity and fitness of 874 older adults with intellectual disabilities (61.4 ± 7.8 years). All-cause mortality was assessed over a 5-year follow-up period. Results: Fitness, but not obesity, was significantly related to survival (HR range of 0.17-0.22). People who were unfit were 3.58 (95% CI = 1.72-7.46) to 4.59 (95% CI = 1.97-10.68) times more likely to die within the follow-up period than people who were fit, regardless of obesity. Conclusion: This was the first study to show that being fit is more important for survival than fatness in older adults with intellectual disabilities. The emphasis should, therefore, shift from weight reduction to improving physical fitness.