Trimethylamine N‑oxide (TMAO) is a metabolite commonly produced by intestinal flora. Elevated levels of TMAO in plasma have been strongly associated with central cognitive impairment‑related diseases such as Alzheimer's disease, Parkinson's disease, age‑related cognitive decline, and perioperative neurocognitive disorders. TMAO has shown potential as a predictive marker for cognitive impairment‑related diseases. This review aims to summarize previous clinical and basic research on TMAO and its connection to cognitive impairment‑related diseases. Additionally, it will explore the potential mechanisms through which TMAO influences these diseases, offering new insights and references for the prevention and treatment of cognitive impairment‑related diseases.