Biomass and CO valorization constitutes a sustainable solution to mitigate global waste accumulation by converting biomass and CO into valuable chemicals and fuels. Among various conversion strategies, photoelectrochemical (PEC) systems have emerged as a promising approach due to their ability to drive redox reactions under mild conditions using solar energy. However, challenges such as poor selectivity, charge recombination, and inefficient light harvesting hinder the widespread adoption of PEC biomass and CO valorization. In efforts to push the concept into the practical realm, modifying the surface of the corresponding photoanodes has emerged as the most viable and effective approach. Acknowledging its importance, in this review, we thoroughly discuss various surface engineering strategies for enhancing and tuning PEC biomass and CO valorization selectivity. We open the discussion by introducing the fundamental principles of PEC processes, system configurations, and the critical role of surface properties in governing reaction pathways. Building on the previous discussions, common surface engineering strategies, particularly surface functionalization, crystal face tuning, defect engineering, and nanostructuring, are systematically reviewed for their ability to tailor surface properties and modulate the electronic structures of photoelectrodes. Crucially, we provide insights into the interplay between photoelectrode design and reaction dynamics responsible for the improvement and tunability of PEC biomass and CO valorization selectivity. By providing a comprehensive overview of recent advancements, this review aims to serve as a valuable resource for guiding future developments in PEC biomass and CO valorization.