BACKGROUND: The burden of mental disorders (MD) in the Western Pacific Region (WPR) remains a critical public health concern, with substantial variations across demographics and countries. AIM: To analyze the burden of MD in the WPR from 1990 to 2021, along with associated risk factors, to reveal changing trends and emerging challenges. METHODS: We used data from the Global Burden of Disease 2021, analyzing prevalence, incidence, and disability-adjusted life years (DALYs) of MD from 1990 to 2021. Statistical methods included age-standardisation and uncertainty analysis to address variations in population structure and data completeness. RESULTS: Between 1990 and 2021, the prevalence of MD rose from 174.40 million cases [95% uncertainty interval (UI): 160.17-189.84] to 234.90 million cases (95%UI: 219.04-252.50), with corresponding DALYs increasing from 22.8 million (95%UI: 17.22-28.79) to 32.07 million (95%UI: 24.50-40.68). During this period, the burden of MD shifted towards older age groups. Depressive and anxiety disorders were predominant, with females showing higher DALYs for depressive and anxiety disorders, and males more affected by conduct disorders, attention-deficit hyperactivity disorder, and autism spectrum disorders. Australia, New Zealand, and Malaysia reported the highest burdens, whereas Vietnam, China, and Brunei Darussalam reported the lowest. Additionally, childhood sexual abuse and bullying, and intimate partner violence emerged as significant risk factors. CONCLUSION: This study highlights the significant burden of MD in the WPR, with variations by age, gender, and nation. The coronavirus disease 2019 pandemic has exacerbated the situation, emphasizing the need for a coordinated response.