Bipolar disorder (BP) in youth is an impairing psychiatric disorder associated with high rates of relapse and recurrence. High rates of psychiatric and medical co-morbidities account for additional illness burden in pediatric BP. The elevated risk of overweight and obesity in this population is of particular concern. One of the likely etiologies for weight gain in youth with BP is use of mood-stabilizing medications. Although these medications can be effective for mood stabilization, excessive weight gain is a common side effect. Obesity is associated with a host of medical problems and is also correlated with worse psychiatric outcomes in BP, rendering the prevention of weight gain in this population particularly clinically relevant. In this article, we describe the rationale and development of a brief motivational intervention for preventing weight gain among youth with BP initiating mood-stabilizing pharmacological treatment and then present a case example illustrating the principles of the intervention.