This study investigates the clinical relevance of the gut microbiome at taxonomic and metabolic levels in anti-CD19 CAR-T cell therapy, both in patients and in a preclinical syngeneic tumor model. B cell lymphoma patients treated with CD19-CAR-T cells exhibited profound intestinal dysbiosis, exacerbated after CAR-T infusion. This dysbiosis was characterized by low bacterial richness, low sMAdCAM-1 and loss of Akkermansia species, associated with resistance to therapy. Mechanistically, oral Akkermansia massiliensis supplementation increased CAR-T cell infiltration into bone marrow, inverted the CD4/CD8 CAR-T ratio, favored Tc1 CD8+ T cell polarization and promoted release of tryptophan-derived indole metabolites, leading to better tumor control. The clinical benefit of Akkermansia spp. supplementation was abolished when CAR-T cells were genetically deficient for the indole receptor, aryl hydrocarbon receptor (Ahr). Ahr-agonistic indoles alone failed to replicate the bacterium's anticancer effects. These findings suggest Akkermansia supplementation could improve CAR-T cell potency in patients with intestinal Akkermansia deficiency.