Bacterial pathogens must avoid clearance by the immune system to establish infection, yet many processes of bacterial immune subversion remain undefined. T cells are a key component of the mammalian immune system and are required for protective immunity against many bacterial pathogens. Salmonella enterica serovar Typhimurium avoid clearance by the host immune system by suppressing T cell responses, yet the mechanisms mediating this immunosuppression are still unknown. We show that S. Typhimurium inhibit T cell responses by producing L-asparaginase II, which catalyzes the hydrolysis of L-asparagine to aspartic acid and ammonia. L-asparaginase II is necessary and sufficient to suppress T cell blastogenesis, cytokine production and proliferation, and to down-modulate expression of the T cell receptor (TCR). Purified L-asparaginase II alters TCR levels and cytokine profiles of T cells in vitro. Furthermore, S. Typhimurium-induced inhibition of T cells in vitro is prevented upon addition of L-asparagine. S. Typhimurium lacking the L-asparaginase II gene (STM3106) are unable to inhibit T cell responses and exhibit attenuated virulence in vivo. L-asparaginases are used to treat acute lymphoblastic leukemia through mechanisms that likely involve amino acid starvation of leukemic cells and these findings indicate that pathogens similarly use L-asparagine depravation to limit T cell responses.