Sugar industries generate substantial quantities of waste biomass after the extraction of sugar water from sugarcane stems, while biomass-derived porous carbon has currently received huge research attention for its sustainable application in energy storage systems. Hence, we have investigated waste sugarcane bagasse (WSB) as a cheap and potential source of porous carbon for supercapacitors. The electrochemical capacitive performance of WSB-derived carbon was further enhanced through hybridization with silicon dioxide (SiO₂) as a cost-effective pseudocapacitance material. Porous WSB-C/SiO₂ nanocomposites were prepared via the in situ pyrolysis of tetraethyl orthosilicate (TEOS)-modified WSB biomass. The morphological analysis confirms the pyrolytic growth of SiO₂ nanospheres on WSB-C. The electrochemical performance of WSB-C/SiO₂ nanocomposites was optimized by varying the SiO₂ content, using two different electrolytes. The capacitance of activated WSB-C was remarkably enhanced upon hybridization with SiO₂, while the nanocomposite electrode demonstrated superior specific capacitance in 6 M KOH electrolyte compared to neutral Na₂SO₄ electrolyte. A maximum specific capacitance of 362.3 F/g at 0.25 A/g was achieved for the WSB-C/SiO₂ 105 nanocomposite. The capacitance retention was slightly lower in nanocomposite electrodes (91.7–86.9%) than in pure WSB-C (97.4%) but still satisfactory. A symmetric WSB-C/SiO₂ 105//WSB-C/SiO₂ 105 supercapacitor was fabricated and achieved an energy density of 50.3 Wh kg⁻¹ at a power density of 250 W kg⁻¹, which is substantially higher than the WSB-C//WSB-C supercapacitor (22.1 Wh kg⁻¹).