<p>The high cost of fish meal has driven the aquaculture industry to explore alternative protein sources, particularly plant-based proteins. However, these alternatives are often deficient in essential amino acids, including threonine, which is vital for optimal growth and health in aquatic animals. This review provides a comprehensive synthesis of current knowledge on the multifunctional roles of threonine in aquatic species, emphasizing its involvement in growth performance, protein synthesis, nutrient digestion, immune function, and antioxidant defense. We highlight recent advancements in understanding threonine’s interaction with other amino acids, particularly methionine and lysine, and its influence on amino acid balance and metabolic crosstalk. The review also examines species-specific threonine requirements and discusses how dietary strategies can be used to optimize threonine levels in formulated feeds. Environmental factors such as water temperature, stocking density, and farming practices that influence threonine utilization are also considered. Unlike previous reviews, this article integrates emerging data on the physiological consequences of both threonine deficiency and excess, offering updated insights into its role in modern aquaculture nutrition. By aligning dietary threonine levels with species needs and environmental conditions, aquaculture operations can enhance feed efficiency, disease resistance, and overall fish performance.</p>