Background/Objectives: Bidirectional communication between the gut and brain is central to neurological and psychiatric health, and abdominal photobiomodulation (PBM) has emerged as a promising non-invasive way to modulate this axis by targeting intestinal mitochondria, epithelial integrity, and the microbiota. We systematically reviewed preclinical and clinical evidence on abdominal PBM, alone or in combined protocols, reporting microbiome, metabolic, or neurobehavioral outcomes.

Methods: Following PRISMA 2020 recommendations, we searched MEDLINE, Scopus, Web of Science, and ScienceDirect through May 2025 for animal and human studies applying PBM to the abdomen and reporting gut-related, metabolic, or brain-related outcomes.

Results: Nine studies met the eligibility criteria (five human, four animal). Human trials, mainly in Parkinson’s and Alzheimer’s disease, used 630–904 nm light and reported gains in mobility, balance, cognition, and olfaction; one trial also showed microbiota modulation with a decreased Firmicutes:Bacteroidetes ratio. Animal models revealed cognitive improvement, reduced neuroinflammation, dopaminergic neuroprotection, and microbial rebalancing. Mechanistic findings converged on enhanced mitochondrial bioenergetics, redox and anti-inflammatory signaling, vagal activation, and short-chain fatty acid-mediated effects.

Conclusions: Current evidence, though limited by small samples, heterogeneous dosimetry, combined treatment sites, and few sham-controlled human trials, suggests that abdominal PBM can influence the gut–brain axis through converging mitochondrial, immune, and microbial mechanisms. Adequately powered randomized trials with standardized dosimetry, validated mechanistic biomarkers, and integrative multi-omics analyses are needed to clarify causal pathways and optimize translational applications.