Phenotypic and functional studies of the programmed death‐1 (PD‐1) molecule on CD4⁺ and CD8⁺ T cells were performed on peripheral blood mononuclear cells from uninfected and simian immunodeficiency virus (SIV)‐infected rhesus macaques. These data demonstrated a rapid upregulation of PD‐1 expression on tetramer‐positive CD8⁺ T cells from MamuA.01⁺ SIV‐infected macaques upon infection. Upregulation of PD‐1 on total CD8⁺ T cells was not detectable. In contrast, CD4⁺ T‐cell PD‐1 expression was markedly higher in total CD4⁺ T cells during chronic, but not acute, infection and there was a correlation between the level of PD‐1 expression on naive and central memory CD4⁺ T cells and the levels of viral loads. Such association was emphasized further by a marked decrease of PD‐1 expression on tetramer‐positive CD8 T cells as well as on CD4⁺ T cells on longitudinal samples collected before and after the initiation of antiretroviral therapy and downregulation of viral replication in vivo. Cloning of PD‐1 and its two ligands from several non‐human primate species demonstrated > 95% conservation for PD‐1 and PD‐L2 and only about 91% homology for PD‐L1. Functional studies using soluble recombinant PD‐1 protein or PD‐1–immunoglobulin G fusion proteins induced marked increases in the SIV‐specific proliferative responses of both CD4⁺ and CD8⁺ T cells from rhesus macaques. The results of these studies serve as a foundation for future in vivo trials of the use of rMamu‐PD‐1 to potentially enhance and/or restore antiviral immune responses in vivo.