Chicken Interferon-Gamma (IFN-γ) is the sole type II interferon in chickens (Gallus gallus) and is produced primarily by activated CD4⁺ and CD8⁺ T lymphocytes as well as natural killer (NK)-like cells, where it plays a central role in cell-mediated (Th1-type) immunity. IFN-γ signals through the IFN-γ receptor complex (IFNGR1/IFNGR2), activating JAK/STAT1 pathways and inducing genes involved in macrophage activation, enhanced antigen presentation via upregulation of MHC class I and II molecules, and promotion of cytotoxic T-cell responses. In poultry health, IFN-γ is critical for defense against intracellular pathogens and economically significant diseases such as avian influenza virus, Newcastle disease virus, Marek’s disease virus, Salmonella spp., Eimeria spp. (coccidiosis), and Mycobacterium avium, where robust Th1 responses contribute to pathogen control and vaccine-induced protection. Altered or insufficient IFN-γ responses can impact disease susceptibility, growth performance, and flock health. In research settings, chickens serve as important models for avian viral oncogenesis (Marek’s disease), mucosal immunity, and vaccine development, and characterization of chicken IFN-γ supports studies of T-cell biology, host–pathogen interactions, immune correlates of protection, and comparative cell-mediated immunity relevant to both veterinary and broader immunological research.