The majority of cancer-related deaths are associated with metastasis; however, little is known about the mechanisms of this process. Hepsin is a cell surface serine protease that is markedly upregulated in human prostate cancer; however, the functional significance of this upregulation is unknown. We report here that hepsin overexpression in prostate epithelium in vivo causes disorganization of the basement membrane. Overexpression of hepsin in a mouse model of nonmetastasizing prostate cancer has no impact on cell proliferation, but causes disorganization of the basement membrane and promotes primary prostate cancer progression and metastasis to liver, lung, and bone. We provide in vivo evidence that upregulation of a cell surface serine protease in a primary tumor promotes cancer progression and metastasis.