Background: There have been some reports on the coexistence of psoriasis and atopic dermatitis; however, the longitudinal relationship between these two diseases remains unclear. Objective: This study aimed to investigate the bidirectional association between psoriasis and atopic dermatitis. Methods: This cohort study recruited patients from the National Health Insurance Research Database in Taiwan. We included 8,206 patients with psoriasis and 32,824 matched controls to assess the risk of atopic dermatitis and 25,743 patients with atopic dermatitis and 102,972 matched controls to assess the risk of psoriasis. Cox regression model was used for the analyses. Results: After adjusting for potential confounders, patients with psoriasis had a higher risk of atopic dermatitis (adjusted hazard ratio [aHR] 13.01; 95% CI 10.23–16.56) than the controls. Patients with atopic dermatitis had a higher risk of psoriasis (aHR 10.37; 95% CI 6.85–15.69) than the controls. Stratified analyses revealed similar results in both sexes and all age groups. Conclusion: Our study demonstrated a bidirectional association between psoriasis and atopic dermatitis, suggesting that psoriasis and atopic dermatitis are not mutually exclusive and may share some biological mechanisms.