Pestanoid A is a rearranged diterpenoid that functions as an osteoclastogenesis inhibitor with an IC50 of 4.2 μM. It can be isolated from the marine-derived fungus Pestalotiopsis sp. (NBUF145). Pestanoid A inhibits NF-kB ligand-induced activation of MAPK and NF-κB signaling pathways by preventing the phosphorylation of ERK1/2-JNK1/2-p38 MAPKs and the nuclear translocation of NF-κB. This compound may be useful in osteoporosis research.