The immune system’s ability to recognize peptides on major histocompatibility molecules contributes to the eradication of cancers and pathogens. Tracking these responses in vivo could help evaluate the efficacy of immune interventions and improve mechanistic understanding of immune responses. For this purpose, we employ synTacs, which are dimeric major histocompatibility molecule scaffolds of defined composition. SynTacs, when labeled with positron-emitting isotopes, can noninvasively image antigen-specific CD8⁺ T cells in vivo. Using radiolabeled synTacs loaded with the appropriate peptides, we imaged human papillomavirus-specific CD8⁺ T cells by positron emission tomography in mice bearing human papillomavirus-positive tumors, as well as influenza A virus–specific CD8⁺ T cells in the lungs of influenza A virus–infected mice. It is thus possible to visualize antigen-specific CD8⁺ T-cell populations in vivo, which may serve prognostic and diagnostic roles.