Kidney fibrosis is a major culprit in the development and progression of chronic kidney disease (CKD), ultimately leading to the irreversible loss of organ function. Thymocyte differentiation antigen-1 (Thy-1) controls many core functions of fibroblasts relevant to fibrogenesis but is also found in a soluble form (sThy-1) in serum and urine. We investigated the association of sThy-1 with clinical parameters in patients with CKD receiving hemodialysis treatment compared to individuals with a preserved renal function. Furthermore, Thy-1 tissue expression was detected in a mouse model of diabetic CKD (eNOS−/−; db/db) and non-diabetic control mice (eNOS−/−). Serum and urinary sThy-1 concentrations significantly increased with deteriorating renal function, independent of the presence of diabetes. Serum creatinine is the major, independent, and inverse predictor of serum sThy-1 levels. Moreover, sThy-1 is not only predicted by markers of renal function but is also itself an independent and strong predictor of markers of renal function, i.e., serum creatinine. Mice with severe diabetic CKD show increased Thy-1 mRNA and protein expression in the kidney compared to control animals, as well as elevated urinary sThy-1 levels. Pro-fibrotic mediators, such as interleukin (IL)-4, IL-13, IL-6 and transforming growth factor β, increase Thy-1 gene expression and release of sThy-1 from fibroblasts. Our data underline the role of Thy-1 in the control of kidney fibrosis in CKD and raise the opportunity that Thy-1 may function as a renal antifibrotic factor.