Canine Programmed Death-Ligand 1 (PD-L1) (CD274, also known as B7-H1) is an immune checkpoint protein that plays a critical role in regulating T cell activation and immune tolerance in dogs (Canis lupus familiaris). PD-L1 is a member of the B7 family of immunoregulatory proteins, which includes molecules such as PD-L2 (CD273), CD80 (B7-1), and CD86 (B7-2) that regulate T cell costimulatory and coinhibitory signaling. PD-L1 is expressed on antigen-presenting cells such as macrophages and dendritic cells, as well as epithelial cells, endothelial cells, and various 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 limit excessive inflammatory responses. In canine health, PD-L1 expression has been reported in several canine cancers, including oral melanoma, osteosarcoma, mast cell tumors, 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 regulate immune responses during chronic infections and inflammatory diseases. Because the PD-1/PD-L1 pathway is a major target of immune checkpoint therapy in human oncology, canine PD-L1 is increasingly studied in comparative oncology and veterinary immunotherapy, supporting development of checkpoint inhibitor therapies and improved understanding of tumor–immune system interactions in dogs.