¹⁸F-THK5351, initially developed as a positron-emission tomography (PET) tracer for tau pathology, was later shown to display high affinity for monoamine oxidase-B (MAO-B), raising uncertainty about the biological origin of its brain signals in neurodegenerative diseases. To resolve this ambiguity, we implemented a multi-scale validation framework integrating enzyme activity inhibition assays, molecular docking, biolayer interferometry, autoradiography, multiple transgenic and viral animal models, and human PET imaging. THK5351 selectively inhibited MAO-B while sparing MAO-A and exhibited reversible binding kinetics to recombinant MAO-B. Computational modelling localized THK5351 near the MAO-B substrate funnel, revealing moderate binding energy and weaker π-π stacking interactions compared with selective tau tracers. Autoradiographic analysis of human cortical tissue demonstrated that tracer binding was dominated by MAO-B-related signals, with a smaller contribution from tau aggregates, a difference insufficient to produce visually distinguishable patterns in clinical imaging. In APP/PS1 mice,¹⁸F-THK5351 uptake colocalized with regions of reactive astrogliosis and was abolished by MAO-B inhibition, whereas overexpression of P301L-hTau induced extensive tau deposition without affecting tracer retention. MAO-B knockout reduced both tracer binding and tau phosphorylation, and viral induction of astrogliosis elevated tracer uptake that was reversed by selective MAO-B blockade. In a patient with corticobasal syndrome, tracer signals decreased during selegiline treatment and reappeared after drug withdrawal, mirroring preclinical pharmacological responses. Collectively, these findings demonstrate that¹⁸F-THK5351 uptake primarily reflects MAO-B-mediated reactive astrogliosis rather than tau pathology, providing mechanistic insight into its signal origin and underscoring the value of cross-scale, multimodal validation in PET tracer development for neurodegenerative disease research.