Abstract: Supercapacitors have garnered significant research interest as advanced energy storage devices due to their remarkable stability, extended cycle life, rapid charge-discharge rates, high power density, and strong environmental adaptability. In recent years, porous carbon materials derived from natural biomass have been widely employed in supercapacitors owing to their cost-effectiveness, eco-friendliness, and renewable nature. In this study, Sargassum fusiforme, a species of seaweed, was used as the primary carbon source, with urea serving as a nitrogen dopant and potassium hydroxide as a chemical activating agent. A one-step carbonization and activation process was utilized to synthesize a nitrogen-enriched porous carbon material characterized by a high specific surface area, well-developed pore structure, and optimized nitrogen content. Among all the prepared seaweed-based carbon materials, the sample synthesized at a carbonization temperature of 800 ℃ with a Sargassum fusiforme to urea mass ratio of 1∶1 (designated as YN₁C-800) exhibited the highest specific surface area, reaching 2 494.48 m²/g. However, experimental results indicated that the optimal electrochemical performance was achieved at a carbonization temperature of 700 ℃ with the same mass ratio. The porous carbon prepared under these conditions, denoted as YN₁C-700, exhibited a specific capacitance of 307.9 F/g at a current density of 0.5 A/g in a three-electrode configuration and a substantial specific surface area of 2 371.07 m²/g. Structural characterization of YN₁C-700 by scanning electron microscopy and Brunauer-Emmett-Teller (BET) analysis revealed a hierarchical porous structure. Furthermore, X-ray photoelectron spectroscopy confirmed the successful incorporation of nitrogen species, with the N 1s spectrum showing a high relative content of N-6 (pyridinic N) and N-5 (pyrrolic N) configurations. To evaluate its practical performance, symmetric supercapacitors were assembled using two different electrolytes: 6 mol/L KOH and 1 mol/L Na₂SO₄. In the 6 mol/L KOH electrolyte, the device based on YN₁C-700 delivered an energy density of 13.37 W·h/kg at a power density of 325.0 W/kg. After 10 000 consecutive charge-discharge cycles, the capacitor retained 70.45% of its initial capacitance. When tested in the 1 mol/L Na₂SO₄ aqueous electrolyte, the supercapacitor demonstrated improved performance, achieving an energy density of 20.78 W·h/kg at a power density of 400.0 W/kg and exhibiting superior cycling stability with a capacitance retention of 83.67% after 10 000 cycles. These findings suggest that the biomass-derived porous carbon material produced via this facile synthetic route holds great promise for application in high-performance supercapacitors, combining competitive electrochemical properties with the advantages of sustainability and low cost. Moreover, the performance differences between the two electrolytes highlight the critical role of electrolyte selection in optimizing device characteristics. This study provides an effective strategy for developing high-performance, low-cost, and environmentally friendly supercapacitor electrode materials, while opening up a new avenue for the high-value utilization of seaweed resources.