Ginsenosides, the major active ingredients of ginseng, have a variety of biomedical efficacies such as anti-aging, anti-oxidation, and anti-inflammatory activities. To understand the effects of compound K (20-O-beta-D-glucopyranosyl-20(S)-protopanaxadiol), one of the major metabolites of ginsenosides, on the skin, we assessed the expression levels of about 100 transcripts in compound K-treated HaCaT cells using cDNA microarray analysis. One gene up-regulated by compound K was hyaluronan synthase2 (HAS2). Semi-quantitative RT-PCR showed that compound K increased HAS2 mRNA in time- and dose-dependent manners. ELISA and immunocytochemistry using hyaluronan (HA)-binding protein showed that compound K effectively increased HA production in HaCaT cells. Finally, treatment of compound K on hairless mouse skin increased the amount of HA in the epidermis and papillary dermis. Our study suggests that topical application of compound K might prevent or improve the deteriorations, such as xerosis and wrinkles, partly ascribed to the age-dependent decrease of the HA content in human skin.