Hydrogels (HGs), three-dimensional cross-linked hydrophilic polymer networks, have emerged as a promising class of functional materials for sustainable agriculture due to their exceptional water retention capacity, responsiveness to environmental stimuli, and favorable biocompatibility. This review systematically summarizes the key functional properties of hydrogels and critically examines their multidimensional roles within agricultural systems. The major synergistic benefits of hydrogels are highlighted, including (1) improvement of soil physical structure, chemical properties, and the biological microenvironment, thereby facilitating soil remediation; (2) direct enhancement of seed germination, root development, and crop productivity when employed as soil amendments or seed-coating materials; (3) controlled and sustained release of water, nutrients (N, P, K, and trace elements), and pesticides, leading to significant improvements in resource use efficiency; (4) functional delivery of beneficial microorganisms, enabling precise regulation of their activity and efficacy; and (5) advancement of soilless cultivation technologies through the development of sophisticated hydrogel-based substrates. Furthermore, this review discusses the key challenges that currently limit large-scale agricultural implementation, including insufficient biodegradability, potential ecotoxicological risks, and techno-economic constraints. Finally, future research directions are proposed from an interdisciplinary perspective, emphasizing rational material design, performance optimization, and practical field application. This comprehensive review aims to provide systematic theoretical guidance and practical insights for the development and deployment of hydrogel-based technologies in sustainable agriculture.

To address the limitations of conventional polymer hydrogels in terms of sustainability and functionality, green soy protein (SP)-based hydrogels (SPHs) demonstrate significant potential. As an abundant, renewable plant protein, SP provides an ideal molecular platform for constructing high-performance, multifunctional hydrogels. This review systematically consolidates recent progress in SPHs. Firstly, the molecular fundamentals and gelation mechanisms of soy protein are analyzed in depth. Subsequently, key construction strategies, including physical, chemical, and enzymatic crosslinking, as well as composite/hybrid approaches, are comprehensively reviewed with respect to their mechanisms, advantages, and limitations. Following this, innovative applications of SPHs in biomedical, food and nutrition, environmental/agricultural, and smart material fields are highlighted. Finally, the current challenges facing research in mechanical properties, structure–property relationships, and scalable production are identified. Future directions include developing novel green crosslinking systems, deepening multi-scale structural control, and advancing smart integrated design. This review aims to provide researchers with a systematic knowledge framework spanning from “molecular understanding” to “functional customization,” thereby propelling soy protein hydrogels toward higher toughness, intelligence, and sustainability.