Excessive bone resorption mediated by mature osteoclasts can cause osteoporosis, leading to fragility fractures. Therefore, an effective therapeutic strategy for anti-osteoporosis drugs is the reduction of osteoclast activity. In this study, the osteoclast inhibitory activity of a novel compound, N-phenyl-methylsulfonamido-acetamide (PMSA), was examined. PMSA treatment inhibited receptor activator of nuclear factor kappa B ligand (RNAKL)-induced osteoclast differentiation in bone marrow-derived macrophage cells (BMMs). We investigated two PMSAs, N-2-(3-acetylphenyl)-N-2-(methylsulfonyl)-N-1-[2-(phenylthio)phenyl] glycinamide (PMSA-3-Ac), and N-2-(5-chloro-2-methoxyphenyl)-N-2-(methylsulfonyl)-N-1-[2-(phenylthio)phenyl]glycinamide (PMSA-5-Cl), to determine their effects on osteoclast differentiation. PMSAs inhibited the signaling pathways at the early stage. PMSA-3-Ac inhibited tumor necrosis factor receptor-associated factor 6 (TRAF6) expression, whereas PMSA-5-Cl suppressed the mitogen-activated protein kinase (MAPK) signaling pathways. However, both PMSAs inhibited the master transcription factor, nuclear factor of activated T cell cytoplasmic-1 (NFATc1), by blocking nuclear localization. An in vivo study of PMSAs was performed in an ovariectomized (OVX) mouse model, and PMSA-5-Cl prevented bone loss in OVX mice. Therefore, our results suggested that PMSAs, specifically PMSA-5-Cl, may serve as a potential therapeutic agent for postmenopausal osteoporosis.