Systemic sclerosis (SSc) is a debilitating inflammatory and fibrotic disease that affects the skin and internal organs. Although the pathophysiology of SSc remains poorly characterized, mononuclear cells, mainly macrophages and T cells, have been implicated in inflammation and fibrosis. Inducible costimulator (ICOS), which is expressed on a subset of memory T helper (T_H) and T follicular helper (T_FH) cells, has been shown to be increased in SSc and associated with disease pathology. However, the identity of the relevant ICOS⁺ T cells and their contribution to inflammation and fibrosis in SSc are still unknown. We show that CD4⁺ ICOS-expressing T cells with a T_FH-like phenotype infiltrate the skin of patients with SSc and are correlated with dermal fibrosis and clinical disease status. ICOS⁺ T_FH-like cells were found to be increased in the skin of graft-versus-host disease (GVHD)–SSc mice and contributed to dermal fibrosis via an interleukin-21– and matrix metalloproteinase 12–dependent mechanism. Administration of an anti-ICOS antibody to GVHD-SSc mice prevented the expansion of ICOS⁺ T_FH-like cells and inhibited inflammation and dermal fibrosis. Interleukin-21 neutralization in GVHD-SSc mice blocked disease pathogenesis by reducing skin fibrosis. These results identify ICOS⁺ T_FH-like profibrotic cells as key drivers of fibrosis in a GVHD-SSc model and suggest that inhibition of these cells could offer therapeutic benefit for SSc.