Phlorotannins are metabolites synthesised by brown algae to protect against environmental stresses. Only a few studies presented structural elucidation of native molecules extracted from macroalgae, contrary to previous published studies, which characterised acetylated phlorotannins. The present work introduces quantitative and qualitative studies to characterise phlorotannins from the eight species of Sargassaceae settled in Brittany, using a colorimetric method used for quantification of total phenolic content (TPC), and one- and two-dimensional nuclear magnetic resonance (NMR) (1H, heteronuclear multiple bond correlation) together with in vivo NMR (high-resolution magic-angle spinning (HR-MAS) NMR) analyses, with the aim to elucidate structural and fingerprint of phlorotannins signals. Halidrys siliquosa, which settles in low tide rock pools, exhibited high TPC (17.77±1.68% dry weight (DW)) while the other species showed lower TPC (ranged from 0.85±0.11% DW for Cystoseira baccata growing in low tide rock pools to 5.53±0.63% DW in Cystoseira humilis growing in high tide rock pools). No relation between TPC and the position of the algae along the shore could be highlighted. Fingerprints using NMR HR-MAS were useful to compare the eight species in terms of phenolic content and in terms of phenolic signals as the species from Brittany produced different phlorotannins. Our study demonstrated that Cystoseira tamariscifolia produced the monomer (phloroglucinol), C. humilis, phloroglucinol and a phlorethol, C. baccata and Cystoseira nodicaulis produced many compounds, i.e. traces of phloroglucinol together with fucols, phlorethols, and fucophlorethols and finally fuhalols were identified in Bifurcaria bifurcata and H. siliquosa. The putative ecophysiological roles of these phlorotannins from Sargassaceae are discussed, together with their potential bioactivities.