Feline Programmed Death-Ligand 1 (PD-L1) (also known as CD274 or B7-H1) is an immune checkpoint protein that plays a key role in regulating T cell activation and immune tolerance in cats (Felis catus). PD-L1 is expressed on antigen-presenting cells, macrophages, dendritic cells, epithelial cells, and some tumor cells, and interacts with its receptor PD-1 (Programmed Death-1, CD279) on activated T lymphocytes. Binding of PD-L1 to PD-1 delivers inhibitory signals that suppress T cell proliferation, cytokine production, and cytotoxic activity, helping to maintain immune homeostasis and prevent excessive inflammatory responses. In feline health, PD-L1 expression has been observed in several feline cancers, including oral squamous cell carcinoma, mammary carcinoma, and lymphoma, where tumor-associated PD-L1 expression can contribute to immune evasion by inhibiting anti-tumor T cell responses. PD-L1 signaling may also play roles in chronic infections and inflammatory diseases where regulation of immune activation is required to limit tissue damage. Because immune checkpoint pathways are increasingly important therapeutic targets in oncology, feline PD-L1 is an emerging focus in comparative cancer research and veterinary immunotherapy, contributing to studies aimed at developing checkpoint inhibitor strategies and understanding tumor–immune system interactions in cats.