Background: Psoriasis is a common inflammatory skin disorder. The typical cutaneous lesions are papules and plaques covered with silvery scales distributed mainly on extensor aspects of extremities. The course and progression of the disease are unpredictable and vary in each patient. YKL-40 is a new inflammatory biomarker which may be involved the pathogenesis of the disease. Objective: The aim of this study was to study YKL-40 expression in patients with psoriasis to evaluate its possible role in the pathogenesis of psoriasis. Patients and Methods: This study included 30 patients with chronic plaque psoriasis and 10 healthy individuals of matched age and sex served as a control group. Punch biopsies of four mm were taken from the skin of psoriatic patients as well as from corresponding sites of control subjects. All specimens were examined by both hematoxylin and eosin stain and anti-YKL-40 stain. Results: The intensity of YKL-40 expression was upregulated in both epidermis and dermis of psoriatic skin in comparison with normal skin of control subjects. YKL-40 expression in psoriatic skin showed significant positive correlation with Psoriasis Area and Severity Index (PASI) score. Conclusion: YKL-40 involved in the pathogenesis of psoriasis manifested by its up-regulation in skin of psoriatic patients group in comparison to control group.