Ergothioneine is a naturally occurring sulfur-containing amino acid derivative, specifically a 2-thioimidazole derivative of histidine. It is characterized by its unique thione-thiol tautomerism and is biosynthesized primarily by certain fungi and mycobacteria, but not by plants or animals, making it a dietary nutrient for humans. Its primary significance lies in its potent antioxidant and cytoprotective properties. Ergothioneine acts as a dedicated antioxidant, being selectively transported into cells and mitochondria via the specific organic cation transporter OCTN1, where it scavenges reactive oxygen and nitrogen species and protects against oxidative stress and cellular damage. This has led to its investigation for potential roles in mitigating age-related diseases, neurodegenerative disorders, and inflammation. Due to its exceptional stability and skin-protective benefits, ergothioneine has gained substantial commercial traction as a high-value active ingredient in the cosmetic and nutraceutical industries. It is marketed for its anti-aging, anti-inflammatory, and UV-protective effects in skincare formulations, as well as a dietary supplement for promoting overall health and longevity.