<p>Aquaculture is experiencing global expansion, heightening the demand for sustainable feed additives that enhance growth, immunity, and disease resistance. <i>Astragalus polysaccharides</i> (APs) possess significant immunomodulatory, antioxidant, and anti-inflammatory properties. This review systematically examines the structural characteristics, biological functions, underlying mechanisms, and physiological effects of APs in aquatic species, with particular emphasis on their immunomodulatory, antioxidant, and anti-inflammatory activities. Furthermore, APs have been shown to promote growth performance in aquatic organisms by enhancing feed utilization efficiency, modulating digestive enzyme activities, and regulating nutrient metabolism. Evidence indicates that APs enhance immune function, modulate gut microbiota, and mitigate environmental stress, especially when combined with probiotics, prebiotics, plant extracts, or vaccines. Species-specific responses of finfish, crustaceans, and other aquatic creatures are compared in terms of dosage, safety, and physiology. The review also discusses extract standardization, regulatory challenges, economic viability, and prospective research directions for optimizing AP formulations in sustainable aquaculture. This work offers academics, aquafeed producers, and industry stakeholders a comprehensive resource for health-oriented and environmentally sustainable aquaculture systems, by combining molecular insights and practical zootechnical results.</p>