Citron (Citrus junos Sieb. Ex Tanaka), a citrus fruit native to Korea, Japan, and China, has been commonly used as a raw material for beverages and herbal medicines. Citron seeds, the by-product of processing industry, contain many bioactive compounds that exhibit several biological properties, such as antioxidative, anti-inflammatory, and antiobesity. However, potential of citron seed oil for hair growth promotion remains unexplored. In the present study, we investigated the hair growth-promoting effect of citron seed oil and its underlying mechanism(s) in a C57BL/6 mouse model. The results show that citron seed oil effectively promotes hair regrowth by early induction of telogen-to-anagen transition and by increasing hair follicle density and hair bulb diameter as well as skin thickness. Citron seed oil treatment increased β-catenin, lymphoid enhancer binding factor 1, sonic hedgehog, cyclin D1 and cyclin E expressions, and alkaline phosphatase activity, compared to the vehicle-treated control mice. Topical administration of citron seed oil upregulated expression of antiapoptotic Bcl-2 protein but did not alter protein level for proapoptotic Bax in mouse skin tissues. Furthermore, insulin-like growth factor 1 and vascular endothelial growth factor were also induced by citron seed oil treatment. These findings suggest the potential of citron seed oil in the prevention and/or treatment of hair loss by inducing hair cycling and regeneration.