In this study, fragile thermoset phenolic-type polymers were chosen as composite matrices that were reinforced with sugarcane bagasse fibers, an agro-residue widely available across the world, which has motivated the search for new applications for this fiber. Lignosulfonate (SL), a macromolecule obtained from lignocellulosic biomass and having phenolic rings in its chemical structure, was used as a substitute for phenol in the synthesis of phenolic-type polymers, yielding lignophenolic polymers. To our knowledge, composites based on lignosulfonates and sugarcane bagasse have not yet been reported in the literature. Composites were also prepared from phenol for a comparison of the properties. SL was also used for the first time to modify the surface of sugarcane bagasse fibers via adsorption, aiming to intensify the interactions at the interface between fibers and matrix, which influences the properties of composites. Untreated and treated (with SL) fibers were characterized for composition and crystallinity, and analyzed using scanning electron microscopy, thermogravimetric analysis, and inverse gas chromatography (IGC). These fibers were used as reinforcements (50 and 70 wt\%) in phenolic and lignophenolic matrices. The impact strength of the composites was determined, and the fractured surfaces (after impact) were characterized using SEM. The IGC results showed that the treatment of the fibers increased the number of acidic groups (mainly owing to the increase in the -OH groups, derived from SL) and electron donors (mainly SO3- groups present in the structure of SL) on the surface of the fibers. Similar results were observed in a previous study on the lignophenolic thermoset, which exhibited a greater amount of both acidic and donor groups compared to the phenolic thermoset. The impact strength of all the composites (phenol- and lignophenolic-based matrices) was considerably higher than that of the unreinforced thermoset. The best results were observed for the lignophenolic matrices reinforced with treated fibers (50 and 70 wt\%). The high amount of acidic and donor groups on the surface of these fibers and in the lignophenolic polymer favored the interactions between the donor sites of the treated fibers and the lignophenolic matrix acceptors and vice versa, leading to a strong adhesion at the interface of the fibers and matrix. This study showed that surface modification of sugarcane bagasse fibers through adsorption could be a promising alternative to modifications using chemical reactions, besides increasing the possibilities of the use of sugarcane bagasse and SL. To our knowledge, the approach of this study is unprecedented.