This study was focused on the phenolic content and associated cosmetic activities of seven brown marine macroalgae:Alaria esculenta,Ascophyllum nodosum,Bifurcaria bifurcata,Fucus serratus,Halidrys siliquosa,Himanthalia elongata, andLaminaria ochroleuca, selected for their abundance in Brittany and their phlorotannin production. Phlorotannins were extracted by maceration followed by a liquid/liquid purification to obtain fractions enriched in phlorotannins. Seasonal and yearly variations in phenolic contents were investigated thanks to the Folin-Ciocalteu assay to find the best harvesting season for each species.H-1 nuclear magnetic resonance (NMR) was used to validate the presence of phenolic compounds and to compare the species in terms of phenolic signals, as they produce different phlorotannin molecules. Activities required for cosmetic applications (i.e., radical scavenging, antioxidant, photoprotective, and anti-aging activities) were assessed by in vitro tests. Fucales, especiallyA. nodosum,F. serratus,H. elongata, andH. siliquosa, showed higher phenolic contents than Laminariales.Temporal variations were apparent for five seaweeds with the highest phenolic content, notably in autumn. The phenolic content was high regardless of the season and antioxidant and photoprotective activities were similar to those of commercial molecules, allowing year-round algal harvesting. The anti-aging activity ofA. nodosumandF. serratusfractions was higher than the active tea flavonol, epigallocatechin gallate. These results are promising for the use of abundant marine macroalgae as a source of natural active ingredients for cosmetic applications.