Pestanoid A is a rearranged pimarane diterpenoid osteoclastogenesis inhibitor with an IC 50 of 4.2 μM. Pestanoid A can be isolated from the marine mesophotic zone chalinidae sponge-associated fungus, Pestalotiopsis sp. NBUF145. Pestanoid A inhibits the receptor activator of NF-kB ligand-induced MAPK and NF-κB signaling by suppressing the phosphorylation of ERK1/2-JNK1/2-p38 MAPKs and NF-κB nuclear translocation. Pestanoid A can be used for the study of osteoporosis.