Background: Psoriatic arthritis (PsA) is a chronic multifaceted seronegative arthritis of inflammatory origin. Plasma YKL-40 is an inflammatory biomarker. Objective: To evaluate the role of Chitinase-3-like protein 1 (YKL-40) as a diagnostic biomarker in psoriatic arthritis (PsA) patients and determine its value in assessment of psoriatic arthritis activity and severity. Patients and Methods: Our study included 25 adult patients with evidence of PsA and twenty-five healthy matching controls. All underwent history, laboratory investigation, clinical examination, assessment of activity and severity according to different scores. Results: We observed the presence of a high statistically significant difference between the patients and controls in terms of the blood levels of YKL-40 with a high sensitivity and specificity for diagnosing PsA. Both the PASI score and the DAPSA score had statistically significant positive associations with the YKL- 40 levels serum levels. Conclusion: YKL-40 measurement may act as a biomarker for diagnosis of PsA, as it has the potential to contribute to the fundamental processes underlying the pathogenesis of PsA. So, we concluded that the addition of YKL-40 as a serological marker has a valuable role in diagnosis and severity assessment of PsA.