This study investigated the role of IL-35 in systemic sclerosis (SSc) patients, focusing on CD4+ T cell response and immunomodulatory cytokine production. By comparing the cytokine levels in healthy donors (HD) and SSc patients using ELISAs, we found a significantly lower plasma IL-35 concentration in the SSc patients (52.1 ± 5.6 vs. 143 ± 11.1,p< 0.001). Notably, the IL-35 levels showed a negative correlation with TGF-β (p< 0.001) and IL-17 (p= 0.04). Assessing the IL-35R expression across cell types in the SSc patients and HDs via flow cytometry, we found higher levels on monocytes (40.7 + 5.7 vs. 20.3 ± 1.9,p< 0.001) and lower levels on CD8+ T cells (61.8 ± 9.2 vs. 83.4 ± 0.8,p< 0.05) in the SSc patients. The addition of recombinant IL-35 to stimulated peripheral blood mononuclear cells reduced the IL-17+CD4+ T cell percentage (9.0 ± 1.5 vs. 4.8 ± 0.7,p< 0.05) and increased the IL-35+CD4+ T percentage (4.1 ± 2.3 vs. 10.2 ± 0.8,p< 0.001). In a Treg:Tresponder cell Sco-culture assay with HD and SSc samples, rIL35 decreased the cell proliferation and levels of IL-17A (178.2 ± 30.5 pg/mL vs. 37.4 ± 6.4 pg/mL,p< 0.001) and TGF-β (4194 ± 777 pg/mL vs. 2413 ± 608 pg/mL,p< 0.01). Furthermore, we observed a positive correlation between the modified Rodnan skin score (mRSS) and TGF-β (p< 0.001), while there was a negative correlation between mRSS and IL-35 (p= 0.004). Interestingly, higher levels of plasmatic IL-35 were detected in individuals with limited disease compared to those with diffuse disease (60.1 ± 8.0 vs. 832.3 ± 4.1,p< 0.05). These findings suggest that IL-35 exhibits anti-inflammatory properties in SSc and it may serve as a marker for disease severity and a therapeutic target.