Microbiota-derived hydrogen sulfide (HS) plays a crucial role in modulating the gut-brain axis, with significant implications for neurodegenerative diseases such as Alzheimer's and Parkinson's. HS is produced by sulfate-reducing bacteria in the gut and acts as a critical signaling molecule influencing brain health via various pathways, including regulating inflammation, oxidative stress, and immune responses. HS maintains gut barrier integrity at physiological levels and prevents systemic inflammation, which could impact neuroinflammation. However, as HS has a dual role or a Janus face, excessive HS production, often resulting from gut dysbiosis, can compromise the intestinal barrier and exacerbate neurodegenerative processes by promoting neuroinflammation and glial cell dysfunction. This imbalance is linked to the early pathogenesis of Alzheimer's and Parkinson's diseases, where the overproduction of HS exacerbates beta-amyloid deposition, tau hyperphosphorylation, and alpha-synuclein aggregation, driving neuroinflammatory responses and neuronal damage. Targeting gut microbiota to restore HS homeostasis through dietary interventions, probiotics, prebiotics, and fecal microbiota transplantation presents a promising therapeutic approach. By rebalancing the microbiota-derived HS, these strategies may mitigate neurodegeneration and offer novel treatments for Alzheimer's and Parkinson's diseases, underscoring the critical role of the gut-brain axis in maintaining central nervous system health.