With the increase in life expectancy, reducing the visible signs of skin aging has become a major issue. A reduction in collagen and hyaluronic acid synthesis by fibroblasts is a feature of skin aging. The green seaweed, Ulva intestinalis, is an abundant and rich source of nutrients, especially proteins and peptides. The aim of this study was to assess the potential cosmetic properties of a protein fraction from Ulva intestinalis (PROT-1) containing 51% of proteins and 22% of polysaccharides, and its enzymatic peptide hydrolysates on human dermal fibroblasts. PROT-1 was extracted using a patented acid- and solvent-free process (FR2998894 (B1)). The biochemical characterization and chromatographic analysis showed a main set of proteins (25 kDa). To demonstrate the anti-aging potential of PROT-1, fibroblast proliferation and collagen and hyaluronic acid production were assessed on fibroblast cell lines from donors aged 20 years (CCD-1059Sk) and 46 years (CCD-1090Sk). PROT-1 induced a significant increase in collagen and hyaluronic acid production per cell, and a reduction in cell proliferation without increasing cell mortality. These effects were reversed after protein hydrolysis of PROT-1, showing the central role of proteins in this promising anti-aging property.