The functional contribution of myofibroblasts in fibrosis is not well understood. Using a new genetic mouse model to track and isolate myofibroblasts, we performed gene expression profiling followed by biological validation to identify HE4 (encoding human epididymis protein 4, also known as WAP 4-disulfide core domain-2 or Wfdc2) as the most upregulated gene in fibrosis-associated myofibroblasts. The HE4 gene encodes for a putative serine protease inhibitor that is upregulated in human and mouse fibrotic kidneys and is elevated in the serum of patients with kidney fibrosis. HE4 suppresses the activity of multiple proteases, including serine proteases and matrix metalloproteinases, and specifically inhibits their capacity to degrade type I collagen. In particular, we identified two serine proteases, Prss35 and Prss23, as HE4 targets with functional relevance in kidney fibrosis. Administration of HE4-neutralizing antibodies accelerated collagen I degradation and inhibited fibrosis in three different mouse models of renal disease. Collectively these studies suggest that HE4 is a potential biomarker of renal fibrosis and a new therapeutic target.