Hydrothermal carbonization (HTC) has emerged as a promising thermochemical technology for sustainable management of organic solid waste. Unlike conventional thermal processes, HTC operates at moderate temperatures (180–250°C) and enables the direct conversion of high-moisture feedstocks without energy-intensive drying. This review critically summarizes recent advances in HTC of diverse organic wastes, including food waste, sewage sludge, agricultural residues, and municipal solid waste. The outstanding advantages of HTC, such as high carbon recovery, improved hydrochar fuel properties, reduced greenhouse gas emissions, and effective nutrient retention, are highlighted. The effects of key operating parameters on hydrochar yield and quality are systematically evaluated. Reaction pathways governing the formation of solid, liquid, and gaseous products are also discussed. Potential applications of hydrochar in energy production, environmental remediation, and soil improvement are presented. Despite these benefits, challenges related to scale-up and process optimization remain. Overall, HTC offers a versatile and sustainable platform for converting organic waste into valuable resources, supporting circular economy strategies.