Osteoarthritis (OA) is a global clinical challenge for which no effective disease modifying agents currently exist. Herein we identify protease-activated receptor-2 (PAR-2) as a novel pathogenic mechanism and potential therapeutic target in OA. Experimental OA was induced in wild-type and PAR-2 deficient mice by sectioning the medial menisco-tibial ligament (MMTL), leading to development of a mild arthropathy. Cartilage degradation and increased subchondral bone formation were assessed as indicators of OA pathology. Four weeks following MMTL section, cartilage erosion and increased subchondral bone formation was evident in wild type mice but substantially reduced in PAR-2 deficient mice. Crucially, therapeutic inhibition of PAR-2 in wild type mice,using either a PAR-2 antagonist or a monoclonal antibody targeting the protease cleavage site of PAR-2, was also equally effective at reducing OA progression in vivo. PAR-2 wasupregulated in chondrocytes of wild-type but not sham-operated mice. Wild type mice showed further joint degradation eight weeks following induction of OA, but PAR-2 deficient mice were still protected. The substantial protection from pathology afforded by PAR-2 deficiency following induction of OA provides proof of concept that PAR-2 has a key role in OA and suggests this receptor as a potential therapeutic target. Osteoarthritis (OA) is a chronic disabling condition currently affecting millions globally 1 with radiological evidence of OA in approximately 80% of the population aged over 65. 2OA is characterised by cartilage degradation and increased subchondral bone formation (osteosclerosis). Despite extensive pathophysiologic investigations, clinical management has not altered significantly and comprises administration of analgesics and non-steroidal anti-inflammatory agents and recourse upon joint failure to arthroplasty. No unifying pathogenetic model exists - suggested hypotheses encompass primary cartilage metabolic dysregulation, enthesial disease together with biomechanical dysregulation. Thus far, no critical checkpoint pathway has been identified that is essential for disease progression and which might by corollary represent a valid, disease-modifying OA therapeutic target. Protease-activated receptor-2 (PAR-2) is a G-protein coupled receptor whose 'tethered' ligand is activated by serine proteases. 3 PAR-2 is present in chondrocytes in cartilage from OA patients 4, and following its activation, matrix metalloproteases (MMPs) are generated. 5 However, these previous observations are associative and do not establish the role of PAR-2 in the pathogenesis of OA. We here sought direct evidence of a causal relationship between PAR-2 expression and cartilage and bone pathology in a murine model of OA.