Objectives To investigate whether training individuals from the personal networks of adults with obesity in the skills of motivational interviewing enhances the anthropometric and psychological outcomes of a cognitive-behavioural weight loss intervention. Methods Adults with obesity (N = 201) were randomised to participate in 26 sessions of cognitive behaviour therapy (CBT) for weight loss either alone (CBT-A) or with the addition of a support person (CBT-SP). Outcomes were assessed at the end of the 12-month intervention and at a follow-up one year later. Results Analyses indicated negligible additive effect for the CBT-SP versus the CBT-A condition, although the quality of the patient's relationship with their support person predicted the anthropometric outcomes. Across conditions, significant improvements were observed for all anthropometric (weight, body mass index, and waist circumference) and psychological (self efficacy, weight-related quality of life, weight satisfaction, and binge eating) variables between baseline and post-treatment, and baseline and the follow-up. Conclusions The benefits of the cognitive-behavioural weight loss program were found to extend to psychological variables. Yet the lack of evidence for the additive benefits of including support people in treatment suggests a need to develop more effective training programs for support people in weight management. Trial registration anzctr.org.au Trial ID: ACTRN12611000509965.