The eelgrass Zostera marina L. has several economic roles, from its earlier usage in the insulation industry to protecting the earth from global warming. In this study, we aimed to discover the cosmetic potential of Z. marina. A methanolic extract of Z. marina showed anti-phototoxicity and anti-melanogenesis activity with an IC_ of 17.5 μM, followed by a phytochemical analysis of its phenolic constituents. Ten compounds (1−10) were isolated by several chromatographic techniques and identified by means of nuclear magnetic resonance spectroscopy (NMR) as well as high-resolution mass spectrometry (HR/MS). The identified compounds are caffeic acid (1), 3,4-dihydroxybenzoic acid (protocatechuic acid) (2), luteolin (3), diosmetin (4), 4-coumaroyl-4′-hydroxyl phenyllactic acid (5), rosmarinic acid (6), caffeoyl-4′-hydroxy-phenyllactic acid (isorinic acid) (7), apigenin 7-O-β-D-glucopyranoside (8), luteolin 7-O-β-D-glucopyranoside (9), and luteolin 7-sulfate (10). This is the first report to identify compounds 5 and 7 from the family Zosteraceae. The isolated compounds were assessed for their anti-aging abilities and were found to exhibit good anti-phototoxicity and anti-melanogenesis activities by increasing the viability of UVB-irradiated HaCaT cells by 6% to 34% and by inhibiting melanin synthesis in B16 melanoma cells by 44% to 65%.