Phlorotannins,　recognized　for　their　efficacy　and　safety　as　natural　tyrosinase　inhibitors,　have　posed　challenges　due　to　their　limited　extraction　efficiency　and　the　elusive　nature　of　their　mechanism　of　action.　Here,　we　developed　an　ultrasonic-assisted　enzymatic　extraction　(UAEE)　protocol　that　significantly　enhanced　phlorotannin　recovery　from　Sargassum　horneri,　achieving　a　total　phenolic　content　(TPC)　of　21.51　mg　PGE/g　DW.　Subsequent　purification　using　solvent　extraction　and　Sephadex　LH-20　yielded　high-purity　alginate　phlorotannins　(SHP),　exhibiting　potent　DPPH　radical　scavenging　and　tyrosinase　inhibitory　activities,　with　IC50　values　of　22.71　and　26.90　μg/mL,　respectively.　Structural　elucidation　based　on　NMR　and　MS　data　suggested　that　SHP　predominantly　comprises　fucophloroethols,　with　a　degree　of　polymerization　ranging　from　3　to　8.　Reciprocal　plot　analysis　indicated　that　SHP　inhibits　tyrosinase　activity　through　a　competitive　inhibition　mechanism,　while　fluorescence　spectroscopy　revealed　a　dynamic　quenching　effect　of　SHP　on　tyrosinase.　Molecular　docking　studies　further　elucidated　that　van　der　Waals　forces　and　hydrophobic　interactions　are　the　primary　forces　governing　the　interaction　between　SHP　and　tyrosinase.　Cellular　experiments　demonstrated　the　significant　inhibitory　effects　of　SHP　on　both　melanin　production　and　tyrosinase　activity　in　B16-F10　cells.　This　research　presents　promising　lead　compounds　for　the　development　of　novel　tyrosinase　inhibitors,　while　also　providing　theoretical　and　methodological　foundations　for　the　comprehensive　utilization　of　Sargassum　horneri.　©　2025　Elsevier　B.V.