Brain-derived neurotrophic factor/TrkB signaling plays important roles in cognitive enhancement by promoting neuroplasticity such as neurogenesis. Evaluation of the TrkB activation would be a potential biomarker for cognitive enhancement although its noninvasive evaluation is quite difficult. We hypothesized that TrkB would be detected in serum extracellular vesicles (EVs) leaked from the brain, which would be a useful biomarker for cognitive enhancement. First, we checked secretion of TrkB-expressing EVs from the neuronal cells Neuro2a transfected with a plasmid encoding a flag tagged mouse TrkB (TrkB-flag). TrkB-flag was detected in EVs isolated from the culture medium. Interestingly, intrahippocampal injection of adeno-associated virus serotype PHP.eB vector encoding TrkB-flag gene (AAV-PHP.eB-mTrkB-flag) in mice showed that TrkB-flag was detected in the serum EVs, implying secretion of TrkB-expressing EVs from the brain to circulating blood. Finally, we analyzed the correlation between TrkB activation in serum EVs and cognitive enhancement in humans administered ergothioneine (ERGO)-containing tablets, which are reported to activate TrkB in mice. Oral administration of the ERGO tablets increased ratio of phosphorylated TrkB to TrkB, an indicator of TrkB activation, in serum EVs, which was correlated with plasma ERGO levels and cognitive enhancement. These findings suggest that TrkB-expressing EVs would be secreted into circulating blood, and its phosphorylated states may help quantitative evaluation of cognitive enhancement.