<p>Tooth blackening, the deliberate modification of teeth to achieve a lustrous black appearance, is well documented in modern Vietnam, but its ancient practice remains elusive. This study applies chemical analyses to Iron Age human teeth from the Dong Xa site in northern Vietnam, dated to approximately 2000 years ago. By integrating compositional results with ethnographic parallels and experimental controls, we identify a diagnostic signature of tooth blackening in ancient samples, pointing to iron salts, likely combined with tannin-rich substances, as the primary active components responsible for the coloring process. This research clarifies both the antiquity and the distinctive techniques of tooth blackening in Vietnam, establishing a non-destructive methodological framework for archaeologically identifying such practices. From its emergence at sites associated with the renowned Dong Son bronze culture, through its continuity in later historical accounts and recent memories, tooth blackening underscores a culturally embedded expression of identity within wider networks of interregional interaction and exchange.</p>