T‐cell activation and the subsequent transformation of activated T cells into T‐cell blasts require profound changes in cell volume. However, the impact of cell volume regulation for T‐cell immunology has not been characterized. Here we studied the role of the cell‐volume regulating osmolyte transporter Taut for T‐cell activation in Taut‐deficient mice. T‐cell mediated recall responses were severely impaired in taut^−/− mice as shown with B16 melanoma rejection and hapten‐induced contact hypersensitivity. CD4⁺ and CD8⁺ T cells were unequivocally located within peripheral lymph nodes of unprimed taut^−/− mice but significantly decreased in taut^−/− compared with taut^+/+ mice following in vivo activation. Further analysis revealed that Taut is critical for rescuing T cells from activation‐induced cell death in vitro and in vivo as shown with TCR, superantigen, and antigen‐specific activation. Consequently, reduction of CD4⁺ and CD8⁺ T cells in taut^−/− mice upon antigen challenge resulted in impaired in vivo generation of T‐cell memory. These findings disclose for the first time that volume regulation in T cells is an element in the regulation of adaptive immune responses and that the osmolyte transporter Taut is crucial for T‐cell survival and T‐cell mediated immune reactions.