In vivo generation of CAR T cells offers a transformative approach to cancer immunotherapy by eliminating the need for ex vivo cell manufacturing. In this study, we demonstrate that systemic delivery of a CD19-targeting CAR viral vector can successfully generate functional CAR T cells within a living host. NSG mice were sequentially engrafted with human CD19⁺ cancer cells, human peripheral blood mononuclear cells (PBMCs), and a CD19 CAR-encoding viral vector engineered with a T cell–specific binder to enable selective transduction of T cells. Mice receiving both PBMCs and the T cell–targeted CAR virus exhibited significant anti-tumor activity, whereas control groups did not, indicating that the therapeutic effect was due to in vivo–generated CAR T cells. Our findings suggest that the viral vector selectively infected human T cells within the host, leading to the in situ production of CAR T cells capable of recognizing and eliminating CD19⁺ tumor cells. These results provide compelling evidence that T cell–targeted viral delivery enables efficient and specific in vivo programming of CAR T cells, representing a promising strategy for off-the-shelf immunotherapy.