Aim: Bipolar disorder is a leading disorder contributing to global disease burden, and bipolar depression often becomes severe and refractory. Therefore, clarifying the pathophysiology of bipolar disorder is an urgent issue. Previous reports suggested that factors, such as affective temperaments and childhood maltreatment, aggravate bipolar depression severity. However, to our knowledge, no reports to date have clarified the interrelationship between the above factors and bipolar depression severity. We here hypothesized that childhood maltreatment worsens bipolar depression severity via increasing affective temperaments. To test this hypothesis, a covariance structural analysis was conducted., Methods: The following information was evaluated for a total of 75 people with bipolar disorder using self-administered questionnaires: demographic characteristics, depressive symptoms (Patient Health Questionnaire-9), history of childhood maltreatment (Child Abuse and Trauma Scale), and affective temperaments (Temperament Evaluation of Memphis, Pisa, Paris, and San Diego Autoquestionnaire). The results were analyzed using covariance structure analysis., Results: A significant indirect effect of childhood maltreatment on bipolar depression severity via increasing affective temperaments was identified, whereas the direct effect of childhood maltreatment was not significant., Conclusion: Our results reveal that affective temperaments can mediate the adverse effects of childhood maltreatment on the severity of bipolar depression., Competing Interests: Jiro Masuya has received personal compensation from Otsuka Pharmaceutical, Eli Lilly, Astellas, and Meiji Yasuda Mental Health Foundation, and grants from Pfizer. Yota Fujimura has received research and grant support from Novartis Pharma, Otsuka Pharmaceutical, Astellas, Dainippon Sumitomo Pharma, and Shionogi. Shinji Higashi has received honoraria from Dainippon Sumitomo Pharma and Novartis Pharma. Takeshi Inoue has received personal compensation from Mochida Pharmaceutical, Takeda Pharmaceutical, Eli Lilly, Janssen Pharmaceutical, MSD, Taisho Toyama Pharmaceutical, Yoshitomiyakuhin, and Daiichi Sankyo; grants from Shionogi, Astellas, Tsumura, and Eisai; and grants and personal compensation from Otsuka Pharmaceutical, Dainippon Sumitomo Pharma, Mitsubishi Tanabe Pharma, Kyowa Pharmaceutical Industry, Pfizer, Novartis Pharma, and Meiji Seika Pharma; and is a member of the advisory boards of Pfizer, Novartis Pharma, and Mitsubishi Tanabe Pharma. Ichiro Kusumi has received personal compensation from Astellas, Chugai Pharmaceutical, Daiichi Sankyo, Dainippon Sumitomo Pharma, Eisai, Eli Lilly, Janssen Pharmaceutical, Kyowa Hakko Kirin, Meiji Seika Pharma, MSD, Nippon Chemiphar, Novartis Pharma, Ono Pharmaceutical, Otsuka Pharmaceutical, Pfizer, Mitsubishi Tanabe Pharma, Shionogi, and Yoshitomiyakuhin; has received research/grant support from AbbVie GK, Asahi Kasei Pharma, Astellas, Boehringer Ingelheim, Chugai Pharmaceutical, Daiichi Sankyo, Dainippon Sumitomo Pharma, Eisai, Eli Lilly, GlaxoSmithKline, Kyowa Hakko Kirin, Meiji Seika Pharma, MSD, Novartis Pharma, Ono Pharmaceutical, Otsuka Pharmaceutical, Pfizer, Takeda Pharmaceutical, Tanabe Mitsubishi Pharma, Shionogi, and Yoshitomiyakuhin; and is a member of the advisory boards of Dainippon Sumitomo Pharma and Tanabe Mitsubishi Pharma. The other authors declare that they have no actual or potential conflicts of interest associated with this study. Yu Tamada received honoraria from Otsuka Pharmaceutical, Dainippon Sumitomo Pharma, and Eisai., (© 2023 The Authors. Psychiatry and Clinical Neurosciences Reports published by John Wiley & Sons Australia, Ltd on behalf of Japanese Society of Psychiatry and Neurology.)