Background <p>Juvenile idiopathic arthritis (JIA) is a heterogeneous complex autoimmune disorder that occurs in childhood and is characterized by chronic joint inflammation. Serum ferritin, an acute phase reactant, is elevated in various inflammatory illnesses, including JIA, where it serves as a significant biomarker in systemic JIA. However, its role as a disease activity marker in other JIA subtypes remains underexplored.</p> Objective <p>This study aimed to assess serum ferritin levels in different subtypes of JIA patients and their associations with disease activity.</p> Materials and methods <p>In this cross-sectional study, which was conducted from February 2022 to February 2023, 51 patients diagnosed with JIA were included after exclusion. The diagnostic ability of the serum ferritin level as a potential disease activity marker in JIA patients was assessed.</p> Results <p>The mean age of the JIA patients was 9.1 ± 3.7&#xa0;years. The serum ferritin level and ESR were the highest among systemic JIA (sJIA) patients. Serum ferritin levels were significantly high in JIA patients (<i>P</i> = 0.001) and those with JIA subtypes, sJIA patients (<i>P</i> = 0.048) and Enthesitis related arthritis (ERA) patients (<i>P</i> = 0.049) with high disease activity. There was a positive correlation between the serum ferritin level and the JADAS 27 score in sJIA patients (r_s = + .614, <i>P</i> = 0.015) and ERA patients (r_s = + .512, <i>P</i> = 0.036). Serum ferritin effectively indicates disease activity in systemic and non-systemic JIA, with optimal cutoffs of 348&#xa0;ng/ml (AUC = 0.889) and 168.2&#xa0;ng/ml (AUC = 0.811), respectively.</p> Conclusion <p>The serum ferritin levels were significantly increased in JIA patients with high disease activity and in systemic JIA patients compared with those in nonsystemic JIA patients and among the subtypes of JIA patients and in sJIA &amp; ERA patients with high disease activity.</p>