A UV-absorbing compound was purified and identified as a novel glycosylated mycosporine-like amino acid (MAA), 13-O-β-galactosyl-porphyra-334 (β-Gal-P334) from the edible cyanobacterium Nostoc sphaericum, known as "ge xian mi" in China and "cushuro" in Peru. Occurrence of the hexosylated derivative of shinorine (hexosyl-shinorine) was also supported by LC-MS/MS analysis. β-Gal-P334 accounted for about 86.5% of total MAA in N. sphaericum, followed by hexosyl-shinorine (13.2%) and porphyra-334 (0.2%). β-Gal-P334 had an absorption maximum at 334nm and molecular absorption coefficient was 46,700 at 334nm. Protection activity of β-Gal-P334 from UVB and UVA+8-methoxypsoralen induced cell damage on human keratinocytes (HaCaT) was assayed in comparison with other MAA (porphyra-334, shinorine, palythine and mycosporine-glycine). The UVB protection activity was highest in mycosporine-glycine, followed by palythine, β-Gal-P334, porphyra-334 and shinorine in order. β-Gal-P334 had highest protection activity from UVA+8-methoxypsoralen induced cell damage followed by porphyra-334, shinorine, mycosporine-glycine and palythine. We also found an antioxidant (radical-scavenging) activity of β-Gal-P334 by colorimetric and ESR methods. From these findings, β-Gal-P334 was suggested to play important roles in stress tolerant mechanisms such as UV and oxidative stress in N. sphaericum as a major MAA. We also consider that the newly identified MAA, β-Gal-P334 has a potential for use as an ingredient of cosmetics and toiletries.