Background The prevalence of obesity among individuals with intellectual and developmental disabilities (IDD) is equal to or greater than the general population. Methods Overweight/obese adults (BMI ≥25 kg/m2) with mild‐to‐moderate intellectual and developmental disabilities were randomized to an enhanced stop light diet (eSLD = SLD + portion‐controlled meals, n = 78) or a conventional diet (CD, n = 72) for an 18 months trial (6 months weight loss, 12 months maintenance). Participants were asked to increase physical activity (150 min/week), self‐monitor diet and physical activity and attend counselling/educational sessions during monthly home visits. Results Weight loss (6 months) was significantly greater in the eSLD (−7.0% ± 5.0%) compared with the CD group (−3.8% ± 5.1%, p < .001). However, at 18 months, weight loss between groups did not differ significantly (eSLD = −6.7% ± 8.3%; CD = 6.4% ± 8.6%; p = .82). Conclusion The eSLD and CD provided clinically meaningful weight loss over 18 months in adults with intellectual and developmental disabilities.