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Background: Accumulating evidence indicates that the gut microbiota can synthesize neurotransmitters as well as impact host-derived neurotransmitter levels. In the past, it has been challenging to decipher which microbes influence neurotransmitters due to the complexity of the gut microbiota. Methods: To address whether a single microbe, <i>Bifidobacterium dentium,</i> could regulate important neurotransmitters, we examined <i>Bifidobacteria</i> genomes and explored neurotransmitter pathways in secreted cell-free supernatant using LC-MS/MS. To determine if <i>B. dentium</i> could impact neurotransmitters in vivo, we mono-associated germ-free mice with <i>B. dentium</i> ATCC 27678 and examined fecal and brain neurotransmitter concentrations. Results: We found that <i>B. dentium</i> possessed the enzymatic machinery to generate γ-aminobutyric acid (GABA) from glutamate, glutamine, and succinate. Consistent with the genome analysis, we found that <i>B. dentium</i> secreted GABA in a fully defined microbial media and elevated fecal GABA in <i>B. dentium</i> mono-associated mice compared to germ-free controls. We also examined the tyrosine/dopamine pathway and found that <i>B. dentium</i> could synthesize tyrosine, but could not generate L-dopa, dopamine, norepinephrine, or epinephrine. In vivo, we found that <i>B. dentium</i> mono-associated mice had elevated levels of tyrosine in the feces and brain. Conclusions: These data indicate that <i>B. dentium</i> can contribute to in vivo neurotransmitter regulation.