This study was conducted to evaluate the effects of Sargassum muticum extract and apo-9'-fucoxanthinone, a principal component of S. muticum, on hair growth. When rat vibrissa follicles were treated with S. muticum extract for 21 d, the hair-fiber lengths for the vibrissa follicles increased significantly. Treatment with the S. muticum extract and the EtOAc fraction of the S. muticum extract markedly increased the proliferation of dermal papilla cells (DPCs) and decreased the 5α-reductase activity. In addition, the EtOAc fraction of the S. muticum extract significantly promoted anagen initiation in C57BL/6 mice. Especially, apo-9'-fucoxanthinone, an active constituent from the S. muticum extract, caused an increase in DPC proliferation and a decrease in 5α-reductase activity. To elucidate the molecular mechanisms of apo-9'-fucoxanthinone on the proliferation of DPCs, we examined the level of various signaling proteins. Apo-9'-fucoxanthinone increased the level of vascular endothelial growth factor receptor-2 (VEGF-R2), Wnt/β-catenin signaling proteins such as phospho(ser9)-glycogen synthase kinase-3β (GSK-3β) and phospho(ser552)-β-catenin, whereas apo-9'-fucoxanthinone did not affect the transforming growth factor-β (TGF-β) signaling proteins such as Smad2/3. These results suggest that apo-9'-fucoxanthinone from S. muticum could have the potential for hair growth with DPC proliferation via the activation of Wnt/β-catenin signaling and the VEGF-R2 pathway.