<p>Growing evidence highlights the microbiota–Gut–Brain Axis (MGBA) as a critical pathway linking diet to neurological health. This review synthesizes current evidence on the complementary, additive, and potentially synergistic interactions between probiotics and plant-derived phytochemicals within the MGBA. While the individual benefits of these dietary components are well established, their combined synbiotic application offers expanded mechanistic breadth through coordinated modulation of microbial ecology, epithelial barrier integrity, immune inflammatory signaling, and neurochemical pathways. Probiotics and phytochemicals interact bidirectionally via microbial biotransformation, enhancing short-chain fatty acid production, reducing endotoxin translocation, and attenuating systemic and neuroinflammation. These effects are further linked to indirect modulation of neurotransmitter systems and neurotrophic signaling relevant to mood regulation, cognitive function, and neurodegenerative processes. Evidence from preclinical and emerging clinical studies supports the relevance of these mechanisms in conditions such as Alzheimer’s disease, Parkinson’s disease, mood disorders, and Autism Spectrum Disorder, although human data remain limited. Overall, this narrative review proposes a mechanistic framework describing how probiotics and plant-derived phytochemicals may interact through complementary microbial, immune, and neurochemical pathways within the Microbiota–Gut–Brain Axis. It also highlights current knowledge gaps and emphasizes the need for well-designed clinical studies to validate their combined therapeutic potential.</p>