BACKGROUND: Health problems related to the lack of bone formation are a major problem for ageing populations in the modern world. As a part of the ongoing trend to develop natural substances that attenuate bone loss in osteoporosis, the effects of the edible brown alga Sargassum thunbergii and its active contents on adipogenic differentiation in 3T3‐L1 fibroblasts and osteoblast differentiation in MC3T3‐E1 pre‐osteoblasts were evaluated. RESULTS: Treatment with S. thunbergii significantly reduced lipid accumulation and expression of adipogenic differentiation markers such as peroxisome proliferator‐activated receptor γ, CCAAT/enhancer‐binding protein α and sterol regulatory element binding protein 1c. In addition, S. thunbergii successfully enhanced osteoblast differentiation as indicated by increased alkaline phosphatase activity along raised levels of osteoblastogenesis indicators, namely bone morphogenetic protein‐2, osteocalcin and collagen type I. Two compounds, sargaquinoic and sargahydroquinoic acid, were isolated from active extract and shown to be active by means of osteogenesis inducement. CONCLUSION: S. thunbergii could be a source for functional food ingredients for improved treatment of osteoporosis and obesity. © 2015 Society of Chemical Industry