Osteoclasts are typically differentiated from monocytes (Mo-OC). A subset of osteoclasts (DC-OC) that are differentiated from dendritic cells (DC) has been reported in the arthritic mice model. However, little information is available on DC-OC in humans. The present study applied both in vitro and in vivo experiments to determine the function and pathological significance of DC-OC. DC-OC were differentiated from human monocyte-derived DC and their bone resorption and antigen-presenting functions were investigated. Synovial tissue samples from patients with rheumatoid arthritis were examined for the presence and characteristics of DC-OC. DC-OC differentiated from DC in the presence of M-CSF and RANKL in vitro were demonstrated to be cathepsin K-positive and TRAP-positive multinucleated giant cells. The DC-OC showed stronger bone resorption ability than monocyte-derived osteoclast (Mo-OC) as observed with the pit formation assay. The DC-OC retained CD11c positivity and expressed costimulatory molecules, unlike Mo-OC. T-cells proliferated when co-cultured with DC-OC, but not with Mo-OC. The addition of abatacept to the cocultures reduced T-cell stimulating activity of DC-OC. Abatacept inhibited the differentiation of monocytes into Mo-OC but did not suppress the differentiation of DC into DC-OC. TRAP-positive and CD86-positive DC-OC were detected in the synovial membranes of rheumatoid arthritis patients but not in patients with osteoarthritis. Human DC-OC demonstrated T-cell stimulating activity in addition to osteolytic activity. We further observed this subset of osteoclasts in the inflammatory synovial membrane of patients with rheumatoid arthritis. Such deviations from normal bone metabolism contribute to the inflammation and bone destruction in chronic inflammatory diseases such as rheumatoid arthritis.