Monobromoindigo is a component of Tyrian purple, a purple–red natural colorant extracted from various species of sea snails, which was possibly first produced by the ancient Phoenicians and has been employed as a symbol of royalty and power by several civilizations over the centuries. Raman spectroscopy has proved to be an effective analytical technique to detect historical dyes, as it allows rapid and accurate identification of unknowns in a nondestructive way. Although other constituents of Tyrian purple have been comprehensively investigated by Raman spectroscopy, the Raman bands of 6‐bromoindigo, a molecule that has been correlated with a specific snail species, Hexaplex trunculus (also known as Murex trunculus), have been reported but not previously assigned. This paper includes a complete assignment of the Raman spectrum of the 6‐bromoindigo isomer, including experimental spectra recorded at 488 and 785 nm, which were compared with those collected from indigo under the same conditions. Theoretical Raman spectra for both molecules were obtained using density functional theory calculations. Copyright © 2011 John Wiley & Sons, Ltd.