Platelets amplify inflammation in arthritis via collagen-dependent microparticle production. This study shows that platelets contribute to inflammatory arthritis by releasing microparticles (MPs) that promote inflammation. Platelet MPs were detected in the synovial fluid of patients with rheumatoid arthritis (RA) and other inflammatory arthritises, but not in osteoarthritis (OA). These MPs are proinflammatory and stimulate cytokine responses from synovial fibroblasts via interleukin-1. Depletion of platelets in mice reduced inflammatory arthritis, indicating their role in disease progression. The collagen receptor glycoprotein VI (GPVI) was identified as a key trigger for platelet MP generation in arthritis. Platelet MPs are formed by budding from activated platelets and are present in high concentrations in RA synovial fluid. These MPs are distinct from intact platelets and are enriched in collagen, which activates GPVI. Platelet MPs stimulate synovial fibroblasts to release inflammatory cytokines such as IL-6 and IL-8, contributing to joint inflammation. The study also shows that GPVI deficiency reduces arthritis severity, confirming the role of GPVI in MP generation and arthritis pathogenesis. These findings highlight the previously unrecognized role of platelets and their MPs in inflammatory joint diseases. The results suggest that targeting GPVI could be a novel therapeutic approach for inflammatory arthritis.Platelets amplify inflammation in arthritis via collagen-dependent microparticle production. This study shows that platelets contribute to inflammatory arthritis by releasing microparticles (MPs) that promote inflammation. Platelet MPs were detected in the synovial fluid of patients with rheumatoid arthritis (RA) and other inflammatory arthritises, but not in osteoarthritis (OA). These MPs are proinflammatory and stimulate cytokine responses from synovial fibroblasts via interleukin-1. Depletion of platelets in mice reduced inflammatory arthritis, indicating their role in disease progression. The collagen receptor glycoprotein VI (GPVI) was identified as a key trigger for platelet MP generation in arthritis. Platelet MPs are formed by budding from activated platelets and are present in high concentrations in RA synovial fluid. These MPs are distinct from intact platelets and are enriched in collagen, which activates GPVI. Platelet MPs stimulate synovial fibroblasts to release inflammatory cytokines such as IL-6 and IL-8, contributing to joint inflammation. The study also shows that GPVI deficiency reduces arthritis severity, confirming the role of GPVI in MP generation and arthritis pathogenesis. These findings highlight the previously unrecognized role of platelets and their MPs in inflammatory joint diseases. The results suggest that targeting GPVI could be a novel therapeutic approach for inflammatory arthritis.