Osteoclasts are differentiated from macrophages by RANKL. On the other hand, proinflammatory cytokines such as TNF and IL-6 directly stimulate macrophages to differentiate into osteoclasts (inflammatory osteoclasts) in a RANKL-independent manner. It has been speculated that the inflammatory osteoclasts contribute to the pathological bone destruction in chronic inflammatory lesions observed in rheumatoid arthritis. Therefore, reducing inflammatory osteoclasts is expected to contribute to the improvement of pathological bone destruction. In this study, we used TNF/IL-6-induced osteoclasts as a model of inflammatory osteoclasts and found spermidine as a molecule inhibiting differentiation of the inflammatory osteoclasts. Spermidine has anti-oxidant and anti-inflammatory properties and various beneficial effects on aging-associated diseases. However, little is known about the effects of spermidine on the differentiation of inflammatory osteoclasts. We found that spermidine had no effect on macrophage viability and inhibited differentiation more potently for inflammatory osteoclasts than for RANKL-induced osteoclasts. The inhibitory effect was found to be partially due to the antioxidant property of spermidine. Spermidine may be effective in the treatment of pathological bone destruction.