Lignin is a promising bio-based reinforcing agent for polymer composites; however, its high surface polarity limits compatibility with nonpolar polymers. In this study, lignin surface modification was conducted using glow discharge electrolysis plasma (GDEP) to systematically evaluate the effect of electrolyte type and concentration on plasma behavior and lignin properties. Sodium sulfate (Na2SO4) and potassium sulfate (K2SO4) electrolytes were employed at concentrations of 0.01, 0.03, and 0.05 M. Plasma characteristics were analyzed through current–voltage (I–V) profiles, electrical power, and hydrogen peroxide (H2O2) production, while modified lignin was characterized using FTIR, TGA/DTA, DSC, and surface wettability measurements. Increasing electrolyte concentration reduced discharge voltages and enhanced discharge current, and H2O2 production. FTIR analysis revealed electrolyte-dependent chemical modification: Na2SO4-treatment increased hydroxyl group intensity, whereas K2SO4 treatment reduced hydroxyl content, indicating decreased polarity. Thermal analysis showed that K2SO4 modified lignin exhibited higher thermal stability, with Tmax valued close to untreated lignin (360°C) and char yields of approximately 30–35%. Surface free energy measurements further confirmed enhanced hydrophobicity for K2SO4-treated lignin, decreasing to 90.92–92.21 mN/m compared to untreated lignin (96.74 mN/m). These results demonstrate that electrolyte selection critically governs plasma-induced lignin modification, with K2SO4 providing lignin surfaces more compatible with nonpolar polymer composites. © 2026 Taylor & Francis Group, LLC.