To discover novel osteoclast-targeting antiosteoporosis leads from natural products, we identified 40 tanzawaic acid derivatives, including 22 new ones (1-8, 14-19, 27-32, 37, and 38), from the South China Sea mangrove-derived fungus Penicillium steckii SCSIO 41025. Penicisteck acid F (2), one of the new derivatives showing the most potent NF-κB inhibitory activity, remarkably inhibited osteoclast generation in vitro. Mechanistically, 2 reduced RANKL-induced IκBα degradation, NF-κB p65 nuclear translocation, the activation and nuclear translocation of NFATc1, and the relevant mRNA expression. NF-κB p65 could be a potential molecular target for 2, which has been further determined by the cellular thermal shift assay, surface plasmon resonance, and the gene knock-down assay. Moreover, 2 could also alleviate osteoporosis in ovariectomized mice by reducing the quantities of osteoclasts. Our finding offered a novel potential inhibitor of osteoclastogenesis and osteoporosis for further development of potent antiosteoporosis agents.