Targeted immunotherapy for T cell acute lymphoblastic leukemia (T-ALL), an aggressive tumor of developing T cell progenitors, is an urgent unmet need, especially for relapsed/refractory disease. Selective T-ALL targeting is challenging due to the shared antigen expression between leukemic and normal T cells. Here we identify the pre-T cell receptor (pre-TCR), a surface receptor essential for T cell development, as a biomarker of leukemia-initiating cells (LICs) in human T-ALL. Loss-of-function genetic approaches demonstrate that pre-TCR signaling is necessary for LIC activity and tumor progression in pre-TCR T-ALL patient xenografts in mice. Furthermore, we demonstrate the specific therapeutic targeting of the pre-TCR with a monoclonal antibody against the invariant pTα subunit of the human pre-TCR, and validate an anti-pTα antibody-drug conjugate in vivo treatment as a potent immunotherapy for inhibiting LIC activity and tumor progression of T-ALL in mice. These findings reveal the suitability of pre-TCR targeting as a promising therapy for the treatment of individuals with relapsed/refractory T-ALL expressing the pre-TCR.