Treatment outcomes related to mental well-being and personal recovery are increasingly seen as important for patients with mental disorders, including bipolar disorder (BD). Positive psychology treatments might improve these outcomes in patients with BD. The objective of this study was to evaluate the effectiveness of an 8-week positive psychology group treatment for euthymic patients with BD as an adjunct to treatment as usual (TAU) compared to TAU alone in a pragmatic randomized controlled trial. Patients with BD were randomized to receive either TAU (n = 43) or the positive psychology treatment in addition to TAU (n = 54). The primary outcome was mental well-being measured with the Mental Health Continuum-Short Form. We found significant medium to large between-group effects in favor of the positive psychology group for mental well-being and personal recovery at posttest. Effects for mental well-being were sustained until 12-month follow-up. No significant improvements in social role participation and psychological symptoms were found. The findings suggest that a positive psychological treatment effective in improving mental well-being and personal recovery in euthymic patients with BD. This project was supported by a grant from the Netherlands Organisation for Health Research and Development (ZonMw) (Grant No: 80-84300-98-81007). Netherlands Trial Register (NTR6729).