Abstract The phenolic fingerprint of the leaves of Posidonia oceanica collected from four different Mediterranean Sea regions was determined using a panel of analytical techniques. It was found to be dominated by caffeic tartrates, along with low amounts of coumaric and ferulic tartrates. Chicoric acid (CA) was unambiguously identified as the major phenolic compound, regardless of the sample or collection site. Caftaric acid (CAF) was the second most abundant phenolic, but in much lower average concentrations. These results contrast with most of the previous works, in which CA was not even mentioned. The dramatic influence of extract handling, as well as the importance of choosing the adequate criteria of identity to standards, was demonstrated. These results show the importance of analyzing the crude extract to avoid some of the major pitfalls in determining the phenolic fingerprints. This is the first report to identify and quantify CAF in the Posidoniaceae and to identify CA as the dominant phenolic in Posidonia leaves. The large accumulation of CA in both living (10.12–12.78 mg g-1 dw) and detrital leaves (2.49–12.11 mg g-1 dw) across large geographical scales is noteworthy. The lack of significant concentrations of flavonoids contrasts with other seagrass genera. Importantly, the phenolic signature of P. oceanica shows significant similarities to those of Cymodocea nodosa and Syringodium filiforme, both belonging to the Cymodoceaceae. These results are discussed in terms of metabolic pathways and phylogenetic relationships.< Réduire