Atopic dermatitis (AD) is the chronic relapsing inflammatory skin disorder that affects both in childhood and adulthood. Mounting evidence indicates that gut dysbiosis contributes to AD via the gut-skin axis. Constipation could result in alteration of the gut microflora. The clinical impact of constipation on AD has not been researched. Therefore, we aim to assess the risk of AD in constipated patients by the longitudinal nationwide population-based cohort study. We collected 87015 people with constipation and 87015 patients without constipation between 1999 and 2013 from the Longitudinal Health Insurance Database, which is the subset of Taiwanese National Health Insurance Research Database. Propensity score analysis was administrated to match age, gender, comorbidities, and medications at a ratio of 1:1. Multiple Cox regression analysis was utilized to evaluate the adjusted hazard ratio of AD. In addition, sensitivity tests and a stratified analysis were conducted. The incidence of AD was 4.7 per 1,000 person-years in the constipation group, which was higher than the rate of 2.2 per 1,000 person-years observed in the non-constipation group. After adjustment for age, gender, comorbidities, corticosteroids, and antihistamine, people with constipation had a 2.11-fold greater risk of AD compared to those without constipation (adjusted hazard ratio [aHR]: 2.11 (95% C.I. 1.98-2.24). In subgroup analyses, people aged 12-19 years had a 2.34-fold higher risk of AD in the constipation cohort (aHR; 95% CI, 1.84-2.98). Moreover, people with constipation had a higher likelihood of AD, regardless of gender, and with or without comorbidities, as well as the usage of corticosteroids, and antihistamines. Constipation is connected with a significantly risk factor of AD. Clinicians should be careful of the possibility of AD in people with constipation. Further study is warranted to investigate the possible pathological mechanisms of underlying this relationship.