Here, we show that biomass derived from waste wood from forest (silver birch trees, Betula pendula) is an excellent starting material for fabricating activated carbon for supercapacitors. The carbon was prepared via hydrothermal carbonization with H3PO4followed by pyrolysis. The effect of sulfur doping on its physicochemical and electrochemical properties was evaluated. The samples were named BCM (biomass carbon material) for non-doped and S-BCM (sulfur–biomass carbon material) for the doped samples. We further show that sulfur doping (with around 7% sulfur content) radically increases these nanoparticles’ performance, leading to higher capacitance and stability. The sulfur doping increased the specific surface area to 2124 m2g–1compared to non-doped (1972 m2g–1), as reflected in the enhancement of the number of micropores. In addition, according to Raman spectroscopy analysis, the sulfur doping increased the structural defects based on the ID/IGvalues (S-BCM = 2.23 and BCM = 1.98). Furthermore, the sulfur doping increased the hydrophilicity of the carbon particles, allowing us to disperse them in water and use layer-by-layer self-assembly to fabricate supercapacitor electrodes with nanometer-layer precision. The assembled S-BCM electrodes exhibited a higher capacitance than those of pristine carbon with the highest values measured at 79.1 F/g at 1 A/g. They also had higher stability with a capacitance retention of 85.3% after 10 000 charge–discharge cycles. Our work shows a promising route for making advanced high-performance electrode materials for supercapacitors using waste byproducts, which is especially relevant for the Nordic hemisphere, to minimize carbon footprint while enabling advanced energy storage devices as we aim at reach a smooth transition toward a fossil-free future.