Significance The clinical efficacy of immunotherapy in cancer patients is limited. We have developed a zebrafish model that allows us to visualize the metastatic cancer cell-killing effect by tumor-infiltrating lymphocytes and chimeric antigen receptor T cells in vivo at the single-cell level. This in vivo model accurately predicts clinical efficacy of T cell-mediated cancer therapy. Our data provide an in vivo platform for validating antimetastatic effects by human T cell-mediated immunotherapy. Based on our findings, it is highly recommended that anticancer effects of T cells isolated from cancer patients or genetically propagated T cells should be evaluated in this in vivo animal model before clinical application. If so, millions of cancer patients might benefit from our work.