The objectives of this study are to assess the levels of serum Interleukin-35 (IL-35) in patients with idiopathic inflammatory myopathies (IIMs) and to evaluate the association between IL-35 levels and IIM-related features. Serum IL-35 was detected in 76 patients with dermatomyositis (DM), 28 patients with polymyositis (PM), 98 disease controls (40 rheumatoid arthritis (RA), 34 systemic lupus erythematosus (SLE), 12 systemic sclerosis (SSc), and 12 sjogren syndrome (SS)), and 43 healthy controls by ELISA. Follow-up was conducted on 34 patients. Serum IL-35 was higher in myositis (PM/DM) patients than in healthy controls (median 76.6 pg/ml [interquartile range (IQR) 57.9-136.2] vs. 29.9 pg/ml (IQR 21.9-65.5), P < 0.001) and disease controls. Serum IL-35 in IIM patients negatively correlated with disease duration moderately (r = -0.35, P < 0.01). Patients with dysphagia had higher IL-35 than those without (median149.35 pg/ml (IQR 87.97-267.32) vs. 70.72 pg/ml (IQR 54.49-123.42), P = 0.001). Cross-sectional correlation analysis showed a weak positive correlation between serum IL-35 and CK (r = 0.293, P = 0.003), moderate positive correlation with erythrocyte sedimentation rate (ESR) (r = 0.304, P = 0.002), serum ferritin (SF) (r = 0.467, P = 0.001) and LDH levels (r = 0.401, P < 0.001). Additionally, serum IL-35 was higher in patients who were positive for anti-HMGCR (median 292.04 pg/ml (IQR 67.9-442.86) vs. 74.66 pg/ml (IQR 57.24-131.32), P = 0.038) and anti-SRP antibody (median 130.33 pg/ml (IQR 88.04-481.28) vs. 73.06 pg/ml (IQR 56.78-134.28), P = 0.009) than in negative patients, respectively. Follow-up study showed that changes in IL-35 levels after treatment correlated with changes in MYOACT scores moderately (r = 0.375, P = 0.029). These data indicate that increased serum IL-35 could act as a disease activity marker and as a risk factor for esophageal involvement in IIM. IL-35 may participate in the pathophysiological processes of IIM, but it still needs further study to confirm.