Biohydrogen has emerged as a promising alternative energy carrier due to its renewability and environmental friendliness. In recent decades, significant progress has been made in biohydrogen production from organic waste, particularly agricultural residues, which represent one of the most abundant renewable resources globally. This chapter provides a comprehensive overview of the latest advances in the biotechnological conversion of agricultural biomass into biohydrogen, with a particular focus on dark fermentation as a sustainable and efficient approach. It begins by addressing the structural complexity of agricultural waste and the associated challenges in microbial conversion. The core mechanisms of dark fermentation are discussed, along with key microbial communities involved and the primary factors affecting biohydrogen yield. The chapter further evaluates pretreatment methods, fermentation processes, and emerging strategies for improving performance. In addition, it highlights the importance of machine learning-based modeling, process integration, techno-economic assessment, and life cycle analysis in advancing the practical application of dark fermentation for biohydrogen production from agricultural biomass. The purpose is to provide the theoretical and technical basis for the valorization of agricultural residue into biohydrogen.