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The high molecular weight and poor solubility of seaweed polysaccharides have limited their function and application. In this study, ultraviolet/hydrogen peroxide (UV/H₂O₂) treatment was used to prepare low-molecular-weight seaweed polysaccharides from <i>Sargassum fusiforme</i>. The effects of UV/H₂O₂ treatment on the physicochemical properties and anti-photoaging activity of <i>S. fusiforme</i> polysaccharides were studied. UV/H₂O₂ treatment effectively degraded polysaccharides from <i>S. fusiforme</i> (DSFPs), reducing their molecular weight from 271 kDa to 26 kDa after 2 h treatment. The treatment did not affect the functional groups in DSFPs but changed their molar percentage of monosaccharide composition and morphology. The effects of the treatment on the anti-photoaging function of <i>S. fusiforme</i> polysaccharides were investigated using human epidermal HaCaT cells <i>in vitro</i>. DFSPs significantly improved the cell viability and hydroxyproline secretion of UVB-irradiated HaCaT cells. In particular, DSFP-45 obtained from UV/H₂O₂ treatment for 45 min showed the best anti-photoaging effect. Moreover, DSFP-45 significantly increased the content and expression of collagen I and decreased those of pro-inflammatory cytokines, including interleukin-1β, interleukin-6, and tumor necrosis factor-α. Thus, UV/H₂O₂ treatment could effectively improve the anti-photoaging activity of <i>S. fusiforme</i> polysaccharides. These results provide some insights for developing novel and efficient anti-photoaging drugs or functional foods from seaweed polysaccharides.