Introduction <p>Mental disorders (MD) and obesity are significant global health concerns, with their interrelationship remaining unclear. While previous research has explored their association, the temporal sequence between these conditions has yet to be fully established. This study aims to investigate the bidirectional relationship between MD and obesity, identifying whether one condition precedes the other and assessing the risk associated with different MD subtypes.</p> Methods <p>This retrospective cohort study utilized data from the Taiwan National Health Insurance Research Database. Patients diagnosed with MD or obesity were included, with those having prior diagnoses of both conditions excluded. A two-year washout period was applied to confirm disease onset sequence. Propensity score matching was conducted at a 1:1 ratio based on gender, age, Charlson Comorbidity Index, urbanization level, and insurance amount. A total of 171,056 participants were included in the final analysis. Cox proportional hazards models were used to assess the risk of developing MD after obesity and vice versa.</p> Results <p>Survival analysis revealed no significant association between obesity and the subsequent development of MD. However, patients with MD had a significantly increased risk of developing obesity (aHR = 1.11, <i>p</i> = 0.03). Among MD subtypes, schizophrenia was associated with the highest risk of obesity (aOR = 2.05, <i>p</i> &lt; 0.01), followed by affective disorders (aOR = 1.42, <i>p</i> = 0.01) and anxiety disorders (aOR = 1.35, <i>p</i> = 0.01).</p> Conclusion <p>This study suggests that MD, particularly schizophrenia, is associated with an increased risk of obesity, whereas obesity does not appear to significantly elevate the risk of MD, indicating a unidirectional association. The findings highlight the need for proactive weight management strategies in MD patients to mitigate obesity-related health risks.</p>