While the gut microbiota’s role in various tumors is increasingly recognized, its impact on brain tumors remains poorly understood. Using a murine model, we tracked microbiota changes via 16S rRNA sequencing during tumor progression and reversed dysbiosis with dietary tryptophan. Tryptophan restored microbial balance, enhanced T cell circulation, improved survival, and boosted immunotherapy efficacy. Among responsive species, Duncaniella dubosii replicated these immunomodulatory effects. Our findings highlight gut microbiota—particularly T cell–modulating taxa—as targets to improve brain tumor immunotherapy.