Abstract Relapsed or refractory (R/R) B-cell acute lymphoblastic leukemia (B-ALL) remains a challenging disease with dismal prognosis. Despite the revolutionary impact of CD19-directed chimeric antigen receptor (CAR19) T-cell therapy, >50% of patients relapse within a year. Both leukemia cell–intrinsic factors favoring immune escape and poor CAR T-cell persistence contribute to clinical failure. Moreover, the expression of immune checkpoint receptors (ICRs) and their ligands within the bone marrow (BM) microenvironment may contribute to leukemia progression and therapy resistance. Here, we characterized the expression of ICRs and their ligands in leukemic blasts, T cells, and mesenchymal stromal cells (MSCs) from B-ALL BM samples at diagnosis and relapse, comparing them with age-matched healthy BM controls. Our findings reveal a significantly upregulated expression of TIM-3 in T cells and its ligand, galectin-9, in both blasts and MSCs throughout disease progression. The expression of galectin-9 in B-ALL blasts and TIM-3 in CAR19 T cells negatively correlates with clinical outcome. Furthermore, we demonstrate that galectin-9 impairs CAR19 T-cell homeostasis and cytotoxicity. Notably, an engineered TIM-3–Fc decoy receptor, delivered either by primary T cells coadministered with CAR19 T cells or via a bicistronic all-in-one CAR19–TIM-3–Fc construct, improved the antileukemia efficacy and persistence of CAR19 T cells in B-ALL xenograft models. Mechanistically, CAR19–TIM-3–Fc T-cell treatment promotes the in vivo expansion of transduced and bystander effector and memory T cells, as determined by spectral flow cytometry. Collectively, these TIM-3–Fc decoy–armored CAR19 T cells offer a promising therapeutic strategy for patients with R/R B-ALL.