Major depression, bipolar disorder, and schizophrenia are common mental illnesses, and their potential association with arrhythmias has long been a focus of clinical and research interest. To explore the possible causal relationship between these mental disorders and arrhythmias, we conducted a bidirectional two-sample Mendelian randomization (MR) analysis using publicly available GWAS summary statistics. The primary analysis employed the inverse-variance weighted (IVW) method, with sensitivity analyses including MR-Egger regression and the weighted median estimator to assess robustness and address potential pleiotropy. Results indicated that genetic predisposition to major depression was associated with increased risks of atrial fibrillation/flutter (IVW: OR = 1.214, 95% CI: 1.092-1.349, p < 0.001), paroxysmal tachycardia (IVW: OR = 1.493, 95% CI: 1.261-1.769, p < 0.001), and atrioventricular block (IVW: OR = 1.257, 95% CI: 1.147-1.377, p < 0.001). Reverse MR also suggested a modest effect of atrioventricular block on depression risk (IVW: OR = 1.045, 95% CI: 1.011-1.079, p = 0.008). In conclusion, from the perspective of genetic liability and using the MR framework, our analysis supports a causal role of major depression in increasing the risk of several arrhythmias and suggests potential bidirectional causal effects between depression and atrioventricular block.