Posidonia oceanica is one of the few marine higher plants endemic to the Mediterranean Sea, considered as an indicator of environment quality in coastal areas. Total phenolic, flavonoid and tannin contents were determined in FP (soluble free phenolic) and BP (insoluble-bound phenolic) extracts obtained from Posidonia oceanica intermediate leaves. Higher phenolic content was obtained in FP (27.71± 2.4 GAE/g DW), but higher flavonoid content and tannin contents were detected in BP (19.16± 1.8 mg QE/g DW, and 3.34± 0.5 mg CE/g DW, respectively). Identification and quantification of phenolic compounds were carried out using RP-HPLC analysis. Protocatechuic, ferulic acids and catechin were the major compounds (constituting 90%) of the identified compounds in FP extract, while in BP extract, quercetin was the main flavonol aglycone (about 95%). Higher DPPH scavenging activity (IC50, 2.5 mg/ml), was measured for BP extract. The study of the antimicrobial activity of FP extract showed that it exhibited potentiating activity of some antibiotics. These findings suggested that Posidonia oceanica could be considered as a natural source for pharmacological products.