Background The proallergic cytokine, thymic stromal lymphopoietin (TSLP) may synergize with T cell–derived CD40 ligand (CD40L) to allow IL-23 production in patients with psoriasis. IL-23 is a central cytokine that mediates the inappropriate immune reaction in patients with psoriasis. Objective The aim of the study was to correlate serum level of TSLP with psoriasis severity. Methods The study was carried out on 53 patients with psoriasis. They were divided into mild, moderate, and severe according to PASI score. The patients’ ages ranged from 10 to 62 years. The patients included 29 males and 24 females. A total of 53 healthy subjects with matched age and sex served as control group. Blood samples were collected from the venous blood of the patients and control subjects then the serum was separated. The serum samples were immediately frozen at -20°C. Serum TSLP was measured by Sandwich Enzyme–linked Immunosorbant Assay (ELISA). Results There was a statistically very highly significant increase ( p <0.001) in serum TSLP levels among the case group (1042.7±812.93) compared to the control group (314.21±220.78). There was also a statistically very highly significant increase ( p <0.001) in serum TSLP levels with increased psoriasis severity estimated by PASI score. Conclusion In this study, we found that serum TSLP is elevated in psoriasis patients and is correlated with disease severity.