Black liquor, a by-product of the pulp and paper industry, is the main source of industrial lignin. Its conventional use as fuel for energy recovery underutilizes its potential and contributes to greenhouse gas emissions. Lignin, an aromatic polymer, can replace fossil fuel-based products like hydrogels, biosensors, and carbon fibers. The phenolic hydroxyl group of lignin is particularly important, as it plays a key role in determining lignin's functionality for various applications, including its antioxidant, adhesive, and polymer-stabilizing properties. Accordingly, this study employed a Box–Behnken design with 29 experimental trials to assess how pH, temperature, residence time, and acid concentration influence the total phenolic content (TPC) of lignin extracted via acid precipitation. Within the examined ranges, TPC values varied from 305.72 to 521.78 mg/g GAE, with the highest TPC achieved at pH 3, 70°C, 20% w/v H₂SO₄, and 1.5 h. In contrast, the lowest TPC occurred at pH 6, 25°C, 10% w/v H₂SO₄, and 1.5 h. A regression model and response surface methodology (RSM) were employed to analyze the data, and the model’s significance was confirmed by ANOVA (p < 0.05), despite a moderate F value (2.7257). The model showed good explanatory power (R² = 0.73, R²-adj = 0.43) and an insignificant lack of fit (p = 0.3704), suggesting it accurately captured the effects of the independent variables on TPC. The linear terms for pH and temperature were statistically significant, indicating that reducing pH and increasing temperature strongly enhance TPC. Interaction profiles and three-dimensional response surfaces revealed that acid concentration and residence time also affected TPC, with optimal conditions identified at 20% w/v H₂SO₄ and 1.5 h. These findings demonstrate that the careful optimization of process parameters can substantially improve lignin’s phenolic properties, thereby guiding more efficient, sustainable strategies for lignin valorization.