Engagement of the T cell antigen receptor (TCR) during antigen presentation initiates a coordinated action of a large number of signaling proteins and ion channels. AHNAK1 is a scaffold protein, highly expressed by CD4⁺ T cells, and is a critical component for calcium signaling. We showed that AHNAK1-deficient mice were highly susceptible to Leishmania major infection. AHNAK1-deficient CD4⁺ T cells responded poorly to TCR stimulation in vitro with low proliferation and low Interleukin-2 production. Furthermore, AHNAK1 deficiency resulted in a reduced calcium influx upon TCR crosslinking and subsequent poor activation of the transcription factor NFAT. AHNAK1 was required for plasma membrane expression of L-type calcium channels α1S (Ca_v1.1), probably through its interaction with the β regulatory subunit. Thus, AHNAK1 plays an essential role in T cell Ca²⁺ signaling through Ca_v1 channels, triggered via TCR activation; therefore, AHNAK1 is a potential target for therapeutic intervention.