This study took advantage of the recent discovery that the EphA4 signaling has anti-catabolic effects on osteoclasts/macrophages/synoviocytes but pro-anabolic effects on articular chondrocytes and sought to develop an EphA4 signaling-based therapeutic strategy for osteoarthritis (OA) using a mouse model of OA/posttraumatic OA (PTOA). The injured joint of C57BL/6J mice received biweekly intraarticular injections of a soluble EphA4-binding ligand (EfnA4-fc) at 1 day after the tibial plateau injury or at 5 weeks post-injury. The animals were euthanized 5 weeks later. The injured right and contralateral uninjured left joints were analyzed for hallmarks of OA by histology. Relative severity was determined by a modified Mankin OA scoring system and serum COMP and CTX-II levels. Tibial plateau injury caused more severe OA in Epha4 null mice than in wild-type (WT) littermates, suggesting a protective role of EphA4 signaling in OA. A prototype strategy of an EphA4 signaling-based strategy involving biweekly injections of EfnA4-fc into injured joints was developed and was shown to be highly effective in preventing OA/PTOA when it was administered at 1 day post-injury and in treating OA/PTOA when it was applied after OA has been established. The efficacy of this prototype was dose- and time-dependent. The effects were not caused by the Fc moiety of EfnA4-fc. Other soluble EfnA ligands of EphA4, ie, EfnA1-fc and EfnA2-fc, were also effective. A prototype of a novel EphA4 signaling-based therapy was developed for OA/PTOA that not only reduces the progressive destruction of articular cartilage but may also promote regeneration of the damaged cartilage.