Dolphin TIM-3 (T cell immunoglobulin and mucin domain-containing protein 3, also known as HAVCR2, hepatitis A virus cellular receptor 2) is an immune checkpoint receptor involved in regulating T cell activation, immune tolerance, and inflammatory responses in dolphins, particularly the bottlenose dolphin (Tursiops truncatus). TIM-3 is a member of the TIM (T cell immunoglobulin and mucin domain) family of immune regulatory proteins, which includes TIM-1, TIM-3, and TIM-4, molecules that help control immune activation and maintain immune homeostasis. In dolphins, TIM-3 is likely expressed on activated T lymphocytes, natural killer (NK) cells, dendritic cells, and macrophages, where it functions as a negative regulator of immune responses. Interaction of TIM-3 with its ligands—such as galectin-9, phosphatidylserine, and HMGB1—delivers inhibitory signals that reduce T cell proliferation, cytokine production, and cytotoxic activity, helping prevent excessive inflammation and tissue damage. In marine mammals, immune checkpoint molecules like TIM-3 may be important for maintaining immune balance in the face of chronic pathogen exposure, environmental stressors, and inflammatory challenges. Because dolphins are considered sentinel species for marine ecosystem health, characterization of dolphin TIM-3 supports research into marine mammal immunology, immune regulation during infection, and the impacts of environmental stress on immune function, contributing to a better understanding of host–pathogen interactions in marine environments.