Purpose <p>This study aimed to explore the causal associations between allergic diseases and neuropsychiatric disorders via bidirectional Mendelian randomization (MR) analysis.</p> Methods <p>This study employed three methodologies for MR analysis, primarily utilizing the inverse variance weighted approach, while supplementing with MR-Egger regression and the weighted median method. Sensitivity analyses encompassed Cochran’s Q test, MR-PRESSO test, MR-Egger regression intercept and leave-one-out analysis, all serving to validate the reliability of the reported findings.</p> Results <p>The forward MR analysis results indicated that bipolar disorder (BD) was positively correlated with allergic asthma (AA) (OR = 1.204) and allergic rhinitis (AR) (OR = 4.647), and depression was positively correlated with AA (OR = 1.141) and atopic dermatitis (AD) (OR = 1.103). The reverse MR analysis outcomes unveiled a negative association between allergic urticaria (AU) and autism spectrum disorder (ASD) (OR = 0.780), a positive correlation between allergic conjunctivitis (AC) and tic disorder (TD) (OR = 1.020). Sensitivity analysis indicated an absence of significant heterogeneity or potential horizontal pleiotropy.</p> Conclusions <p>AA and AD acted as risk factors for depression; AA and AR increased the risk of BD; ASD served as a protective factor for AU; TD was a risk factor for AC.</p>