Introduction Autoimmune mechanisms with evident genetic background are the main components of alopecia areata (AA) pathogenesis. Interleukin 15 (IL-15) is considered as an important signalling cytokine. Its disordered expression has been linked to inflammatory autoimmune disorders. Aim The present study aimed to evaluate serum IL-15 in active AA patients and to assess its association with patients’ sex, age, and disease severity. Material and methods IL-15 serum level was measured in 40 patients with active alopecia areata and 20 healthy controls using the ELISA technique. The severity of hair loss was assessed in accordance with the Severity of Alopecia Tool (SALT). Results A significantly higher serum level of IL-15 in AA patients than in controls was detected ( p < 0.001). A significant positive correlation was detected between the SALT score and IL-15 serum level ( r s = 0.433, p = 0.005). No significant correlation between age of the patients and the serum level of IL-15 was observed ( r s = 0.224, p = 0.164). No significant difference in IL-15 serum level regarding patients’ sex, history of disease recurrence, or family history of AA was noted. Conclusions The elevated serum level of IL-15 in active AA patients might reflect its role in disease pathogenesis as a key signalling cytokine. Its level is correlated with disease severity. However, IL-15 is not influenced by patients’ gender or age.