Background An increasing body of observational studies has indicated a potential link between allergic diseases, namely atopic dermatitis (AD), allergic rhinitis (AR), allergic asthma (AA), and psoriasis (PSO) as well as psoriatic arthritis (PSA). However, the presence and causal direction of this association remain uncertain. Methods We conducted two-sample Mendelian randomization (TSMR) analyses utilizing summary statistics derived from genome-wide association studies (GWAS) consortia. The summary statistics were obtained from a substantial participant cohort, consisting of 116,000 individuals (21,000 AD cases and 95,000 controls), 462,933 individuals (26,107 AR cases and 436,826 controls), and 140,308 individuals (4859 AA cases and 135,449 controls). The summary statistics for PSO (9267 cases and 360,471 controls) and PSA (3186 cases and 240,862 controls) were sourced from the FinnGen database. The primary analytical approach employed inverse variance weighting (IVW) as the main method within TSMR. We validated our findings through a series of sensitivity analyses. Furthermore, we performed reverse TSMR analyses to evaluate the potential presence of reverse causality. Results Our investigation revealed a potential protective effect of AD against both PSO (OR = 0.922, 95% CI = 0.863-0.984, p = 0.015)and PSA(OR = 0.915, 95% CI = 0.843-0.993, p = 0.033). Moreover, employing inverse MR analysis, we obtained compelling evidence supporting the protective role of PSO in preventing AD (OR = 0.891, 95% CI = 0.829-0.958, p = 0.002), as well as AR (OR = 0.998, 95% CI = 0.996-0.999, p = 0.008), these associations remained statistically significant even after Bonferroni correction was applied to account for multiple comparisons. Furthermore, our findings did not reveal any substantial causal relationship between AA and either PSO or PSA. Conclusion Our study provides compelling evidence that PSO significantly confers protection against both AD and AR, while AD is likely to act as a protective factor for both PSO and PSA. Despite previous studies suggesting an association between allergic diseases and the incidence of PSO and PSA, our findings do not support this claim. To obtain more accurate and reliable conclusions regarding the causal mechanisms involved, larger sample sizes in randomized controlled trials or MR studies are warranted.