Type II transmembrane serine protease (TTSP) Hepsin belongs to a family of cell-‐surface serine proteases, which have sparked interest as therapeutic targets because of the accessibility of extracellular protease domain for inhibitors. Hepsin is frequently overexpressed in epithelial cancers and elevated hepsin levels promote tumorigenesis in many experimental in vitro and in vivo models. For example, hepsin is consistently overexpressed in more than 40% of examined breast cancers, including all major biological subtypes. However, it is not clear why enhanced hepsin expression is oncogenic while the endogenous expression is not. To explore how elevated hepsin expression confers oncogenicity in breast cancer, a doxycycline-‐inducible hepsin overexpression model was engineered in non-‐transformed mammary epithelial organoids. Induced hepsin acutely downmodulated its cognate inhibitor HAI-‐1, which associated with a sharp increase in pericellular serine protease activity. The derepressed hepsin proteolytically activated downstream serine proteases, augmented HGF/MET signalling and caused deterioration of desmosomes and hemidesmosomes; structures important for cell cohesion and cell-‐basement membrane interaction. Moreover, chronic induction of hepsin considerably shortened the latency of Myc-‐dependent tumourigenesis in the mouse mammary gland. The serine protease and uPA system inhibitor WX-UK1, which is a micromolar range hepsin inhibitor, prevented hepsin from augmenting HGF/MET signalling and disrupting desmosomes and hemidesmosomes. The findings suggest that the oncogenic activity of hepsin arises not only from elevated expression level but also from depletion of HAI-1, events which together trigger gain-of-function activity impacting HGF/MET signalling and epithelial cohesion. Thus, hepsin overexpression is a major oncogenic conferrer to a serine protease activity involved in breast cancer dissemination.