Samples of the edible seaweed, sea mustard (Undaria pinnatifida) were extracted with 80% aqueous methanol, and the concentrated extract was partitioned with ethyl acetate, n-butanol, and water, successively. Isolated fucosterol (1) from the ethyl acetate layers using repeated silica gel and octadecyl silica gel ($C_{18}$, ODS-A) column chromatography and identified it using spectroscopic data including NMR, MS, and infrared spectroscopy (IR). Osteoclasts are specialized multinucleated cells with bone resorbing activity. The induction of c-Fos and NFATc1 transcription factors by receptor activator of nuclear factor ${kappa}B$ ligand (RANKL) is a key event in osteoclast differentiation. Isolated and identification compound 1 was investigated in vitro for their anti-osteoclastic activity. Compound 1 inhibited osteoclastic differentiation in a dose-dependent manner without cytotoxicity. In addition, the inhibitory mechanisms of compound 1 were investigated. Compound 1 induced a clear reduction in the expressions of NFATc1 and c-Fos upon RANKL stimulation. In summary, compound 1 from sea mustard showed anti-osteoporotic potential.