Objective: In cross-sectional studies, elevated body mass index (BMI) is associated with cognitive impairment in bipolar disorder (BD). We investigated the direction of this association by prospectively examining changes in BMI and cognition. Method: We measured BMI and performance in six cognitive domains over 12 months in 80 adolescent and young adult BD patients and 46 healthy comparison subjects (HS). Ninety-three percent of patients received pharmacotherapy and 84% were euthymic. We used repeatedmeasures ANCOVA and longitudinal mixed models to investigate whether (i) higher BMI and increasing BMI over time predicted lower subsequent cognitive functioning, and (ii) lower cognitive functioning and changes in cognition predicted increasing BMI. Results: Neither baseline BMI nor BMI change predicted lower cognitive functioning. Lower baseline scores in attention, verbal memory, working memory, and a composite measure of global cognition predicted increasing BMI in patients and HS. In patients, lower cognitive functioning remained associated with increasing BMI when clinical and treatment variables were adjusted for. Improvement in working memory predicted a smaller subsequent BMI increase in patients. Conclusion: Lower cognitive functioning in specific domains predicts increasing BMI in patients with BD and healthy young adults. Targeting cognition may be important for minimizing weight gain in BD.