Conversion of lignocellulosic residue to bioenergy and biofuel is a promising platform for global sustainability. Fractionation is an initial step for isolating lignocellulosic components for subsequent valorization. The aim of this research is to develop the solvothermal fractionation of sugarcane bagasse to produce high purity lignin. The physio-chemical structure of isolated lignin from this process was determined. In this study, a central composite design-based response surface methodology (RSM) was used to optimize an acid promoter for isolating lignin from sugarcane bagasse using a solvothermal fractionation process. The reaction was carried out with sulfuric acid, at a concentration of 0.01-0.02 M and a reaction temperature of 180-200 degrees C for 30-90 min. The optimal conditions for the experiment were obtained at the acid concentration of 0.02 M with a temperature of 200 degrees C for 90 min in methyl isobutyl ketone (MIBK)/methanol/water (35\% : 25\% : 40\% v/v\%). The results showed that 88\% of lignin removal was done in the solid phase, while 87\% of lignin recovery was conducted in the organic phase. Furthermore, the changes in the physico-chemical characteristics of solid residue and lignin recovery were analyzed using various techniques. GPC analysis of recovered lignin from the organic fraction showed a lower M-w (1374 g mol(-1)) and polydispersity index (1.75) compared to commercial organosolv lignin. The major lignin degradation temperature of commercial organosolv lignin was estimated to be 410 degrees C, whereas BGL showed two main degradations at 291 degrees C and 437 degrees C, which could point to potential relationships with the degradation of beta-O-4 cross-links. The results indicated that recovered lignin was mostly cross-linked by beta-O-4 cross-links. In addition, Py-GC/MS and 2D HSQC NMR gave more information regarding the compositional and structural features of recovered lignin. The development of the sulfuric acid catalyzed solvothermal process in this study provides efficient extraction of high-value organosolv lignin from sugarcane bagasse and the production of recovered lignin in the organic phase with low contamination from other contents. The lignin characteristic data can contribute to the development of lignin valorization in value-added applications.