The strength of the treatment alliance between patients and their clinicians may play a unique role in the management of bipolar disorder. However, few empirical studies have examined the alliance in bipolar disorder or its effects on patient outcomes. This study investigates variables associated with a strong treatment alliance in bipolar disorder, and the prospective effects of treatment alliance on patients' mood symptoms and treatment attitudes. Participants were 58 longitudinally followed individuals with Bipolar I disorder. We found that alliance ratings covaried with depressive symptoms, such that alliance strength increased as depressive symptoms decreased, and stronger alliances were associated with more social support. Tests of temporal association indicated that stronger alliances predicted fewer manic symptoms 6 months later. Stronger alliances also predicted less negative attitudes about medication and less of a sense of stigma about bipolar disorder. Thus, a strong treatment alliance may help to reduce manic symptoms over time. It may be that a strong treatment alliance encourages patients' greater acceptance of bipolar disorder and psychopharmacological interventions, and thus contributes to improved medication adherence and clinical outcomes. Considered in sum, these findings suggest that the treatment alliance is an integral component of the long-term management of bipolar disorder.