Objective To explore the expression of thymic stromal lymphopoietin (TSLP) in patients with diffuse cutaneous systemic sclerosis (dcSSc) and compare its effects in vivo and in vitro with those of interleukin-13 (IL-13) and transforming growth factor β (TGFβ). Methods Skin biopsy specimens from patients with dcSSc (n = 14) and healthy controls (n = 13) were analyzed by immunohistochemistry and immunofluorescence for TSLP, TSLP receptor, CD4, CD8, CD31, and CD163 markers. Wild-type, IL-4Rα1-, and TSLP-deficient mice were treated with TGFβ, IL-13, poly(I-C), or TSLP by osmotic pump. Human fibroblasts and peripheral blood mononuclear cells (PBMCs) were stimulated with TGFβ, IL-13, poly(I-C), or TSLP. Microarray analysis and quantitative polymerase chain reaction were performed to determine gene expression, and protein levels of phospho-Smad2 and macrophage marker CD163 were tested. Results TSLP was highly expressed in the skin of dcSSc patients, more strongly in perivascular areas and in immune cells, and was produced mainly by CD163+ cells. The skin of TSLP-treated mice showed up-regulated clusters of gene expression that overlapped strongly with those in IL-13- and TGFβ-treated mice. TSLP up-regulated specific genes, including CXCL9, proteasome, and interferon (IFN)-regulated genes. TSLP treatment in IL-4Rα1-deficient mice promoted similar cutaneous inflammation as in wild-type mice, though TSLP-induced arginase 1, CCL2, and matrix metalloproteinase 12 messenger RNA levels were blocked. In PBMCs, TSLP up-regulated tumor necrosis factor α, Mx-1, IFNγ, CXCL9, and mannose receptor 1 gene expression. TSLP-deficient mice treated with TGFβ showed less fibrosis and blocked expression of plasminogen activator inhibitor 1 and osteopontin 1. Poly(I-C)-treated mice showed high levels of cutaneous TSLP. Conclusion TSLP is highly expressed in the skin of dcSSc patients and interacts in a complex manner with 2 other profibrotic cytokines, TGFβ and IL-13, strongly suggesting that it might promote SSc fibrosis directly or indirectly by synergistically stimulating profibrotic genes, or production of these cytokines.