The aim of the current study was to investigate the antioxidant ability of Stellaria aquatica L. methanol extract (SAME) and its photoprotection efficacy of B16F10 cells irradiated with UVA. Total polyphenol and flavonoid contents in SAME were estimated to be 284.4 TAE mg/g and 210.5 RE mg/g, respectively. At 1,000 µg/mL, the electron-donating ability of SAME was found to be 58.0% and its SC50 for free radical scavenging was 748.2 µg/ mL. The viabilities of B16F10 cells treated with SAME at 25 and 50 µg/mL were 87.7% and 73.1%, respectively.On the other hand, the viabilities of B16F10 cells treated with IBMX at 25 and 50 µg/mL were 81.3% and 76.1%, respectively. The maximum permissible levels for treating the B16F10 cells with SAME and IBMX were calculated to be 25 µg/mL. Irradiation with 5 and 10 mJ/cm2 UVA decreased the viabilities of the B16F10 cells by 21.6% and 25.1%, respectively, which, however, increased viabilities by 13.0% (p < 0.001) and 11.5% (p < 0.001), respectively on treatment with 25 µg/mL SAME. On the other hand, when 5 and 10 mJ/cm2 UVA-irradiated B16F10 cells was treated with 25 µg/mL IBMX, the viabilities increased by 8.7% (p < 0.05) and 4.7% (p < 0.01), respectively. These results suggest that SAME can be used as a natural cosmetic ingredient for photoprotection of melanocytes from UVA irradiation.