Triggering receptor expressed on myeloid cells 2 (TREM2) is an immune receptor that plays a vital role in innate immune responses. This study aims to investigate the effect of TREM2 on synovial barrier homeostasis and synovitis during temporomandibular joint osteoarthritis (TMJOA). The expression level of TREM2 is decreased in the synovium of both patients with TMJOA and a mouse model of TMJOA, accompanied by synovial barrier breakdown. TREM2 overexpression inhibits the macrophage inflammatory response ex vivo and relieves synovial inflammation, cartilage degeneration, and synovial barrier destruction in monosodium iodoacetate-induced TMJOA mice. RNA-seq analysis reveals that Siglec1 serves as a downstream signal that is downregulated after TREM2 activation. Further in vivo and in vitro experiments demonstrate that rhSiglec1 treatment promotes the synthesis and release of inflammatory cytokines, such as interleukin-6 and RANTES, in macrophages and reverses the alleviation effect of TREM2 activation on TMJOA synovial barrier disorders, synovial inflammation, cartilage degradation, and bone destruction. Overall, this study verifies that TREM2 activation alleviates TMJOA pathology by maintaining synovial barrier homeostasis and inhibiting synovial inflammation. These findings provide new insight into the mechanism of TREM2 in the pathogenesis of TMJOA.