The invention relates to the use of an extract of the alga Phaeodactylum, particularly the alga Phaeodactylum tricornutum, as a cosmetic agent promoting the proteasome activity of skin cells, particularly keratinocytes, fibroblasts or melanocytes (preferably human).The invention makes it possible to manufacture a cosmetic composition for protecting the skin from the adverse effects of UV exposure or for preventing and/or delaying skin ageing effects.