Background: Reducing obesity through effective behaviour change interventions is of key importance to prevent disabling and life-threatening conditions, particularly in individuals already at risk for morbidity. Purpose: To assess the effects of behavioural interventions for obese adults with additional risk factors for morbidity on behaviour, weight and disease risk factors. Methods: Systematic review of randomised controlled trials (RCTs). Three electronic databases and three journals were searched for behavioural interventions (aimed at changing dietary intake and/or physical activity (PA)) for adults (mean BMI ≧30 kg/m²; mean age ≧40 years) with risk factors for morbidity, reporting follow-up data ≧12 weeks. Results: 44 RCTs met the inclusion criteria. Behavioural outcomes, weight loss, and cardiovascular disease risk factors showed consistent modest improvements over time, especially for interventions targeting both diet and PA. Conclusion: Behavioural interventions in at-risk populations showed positive effect tendencies on behaviour, weight, and disease risk factors. However, there is still ample room for improvement, and future research should focus on identifying the most effective means of inducing dietary and PA behaviour change in this vulnerable population.