<p>Aquaculture is developing in many countries to meet the rising protein demand associated with a rapidly growing human population. However, intensification of production brings with it challenges such as deteriorating water quality and increasing disease outbreaks. These conditions lead to an increased risk of infectious diseases and led to higher mortality rates, which causes economic losses. Lactococcosis, particularly that caused by the Gram-positive bacterium <i>Lactococcus garvieae</i>, is one of the most common of these problems and can cause significant losses in various fish species. The intensive use of antibiotics for control purposes creates additional risks in terms of antimicrobial resistance, environmental pollution, and food safety. Therefore, natural feed additives have gained importance in recent years. Certain additives that support immune responses and increase disease resistance in fish have become prominent. Among these, phytobiotics, probiotics, prebiotics, and synbiotics are the most well-known and effective. Their widespread availability, lower cost, and environmental safety make these additives considered an alternative approach to aquaculture. Studies show that these additives strengthen both innate and adaptive immune responses, reduce infection severity, and reduce mortality associated with <i>L. garvieae</i> infections. However, there are still gaps in knowledge regarding how these substances regulate mechanisms such as the immune system, inflammatory processes, antioxidant defenses, and interactions with pathogens. This review aims to clarify these mechanisms by bringing together scientific data obtained in recent years. It also discusses how the information obtained can contribute to the development of safer feed additive strategies and the development of new vaccine approaches. This aims to support the establishment of a more sustainable production structure in the aquaculture sector.</p>