Salmon APRIL (A Proliferation-Inducing Ligand), also known as TNFSF13, is a member of the tumor necrosis factor (TNF) superfamily that plays a role in B cell survival, plasma cell differentiation, and regulation of humoral immunity in salmonid species (e.g., Atlantic salmon, Salmo salar). APRIL is expressed primarily by myeloid-lineage cells and lymphoid tissues, including the head kidney and spleen, which serve as major immune organs in teleost fish. Through interaction with receptors homologous to mammalian TACI and BCMA, salmon APRIL contributes to B cell maturation, antibody production, and maintenance of antibody-secreting cells. In teleost fish, APRIL is particularly important for mucosal immunity, supporting IgM and IgT/IgZ responses in gill, gut, and skin tissues that act as primary barriers against aquatic pathogens. Expression of APRIL may increase during bacterial, viral, or parasitic infections, as well as following vaccination. In aquaculture and comparative immunology research, salmon APRIL is studied in the context of vaccine efficacy, resistance to pathogens such as Aeromonas salmonicida, infectious salmon anemia virus (ISAV), and sea lice infestation, and for its role in adaptive immune regulation in teleost fish. Characterizing APRIL function in salmon provides valuable insight into the evolution of B cell–mediated immunity and supports strategies to enhance disease resistance in aquaculture systems.