Abstract Sialic acid‐binding immunoglobulin‐type lectins (Siglecs) are a family of immunoglobulin‐type lectins that mediate protein‐carbohydrate interactions via sialic acids attached to glycoproteins or glycolipids. Most of the CD33‐related Siglecs (CD33rSiglecs), a major subfamily of rapidly evolving Siglecs, contain a cytoplasmic signaling domain consisting of the immunoreceptor tyrosine‐based inhibitory motif (ITIM) and immunoreceptor tyrosine‐based switch motif (ITSM) and mediate suppressive signals for lymphoid and myeloid cells. While most CD33rSiglecs are expressed by innate immune cells, such as monocytes and neutrophils, to date, the expression of Siglecs in human T cells has not been well appreciated. In this study, we found that Siglec‐5, a member of the CD33rSiglecs, is expressed by most activated T cells upon antigen receptor stimulation. Functionally, Siglec‐5 suppresses T cell activation. In support of these findings, we found that Siglec‐5 overexpression abrogates antigen receptor induced activation of NFAT and AP‐1. Furthermore, we show that GBS β‐protein, a known bacterial ligand of Siglec‐5, reduces the production of cytokines and cytolytic molecules by activated primary T cells in a Siglec‐5 dependent manner. Our data also show that some cancer cell lines express a putative Siglec‐5 ligand(s), and that the presence of soluble Siglec‐5 enhances tumor‐cell specific T cell activation, suggesting that some tumor cells inhibit T cell activation via Siglec‐5. Together, our data demonstrate that Siglec‐5 is a previously unrecognized inhibitory T cell immune checkpoint molecule and suggest that blockade of Siglec‐5 could serve as a new strategy to enhance anti‐tumor T cell functions.