Significance After eradication of smallpox, zoonotic orthopoxviruses, including cowpox virus (CPXV) and monkeypox virus, continue to be a source for infection in humans. Poxviruses dedicate a significant part of their genome encoding proteins that effectively undermine host immune defenses. Here, we demonstrate that a CPXV-encoded protein, M2, can specifically bind B7.1 (CD80) and B7.2 (CD86), cell surface proteins expressed mainly by professional antigen-presenting cells that serve as ligands of the T cell costimulatory receptor CD28 and inhibitory receptor CTLA4. Functionally, we found that M2 blocks CD28-mediated T cell activation in vitro and in a mouse model of cowpox infection.