Phlorotannins are polyphenolic compounds mostly found in brown seaweed and are comprised of polymeric chains of phloroglucinol residues (1,3,5-trihydroxybenzene) connected via C–C and/or C–O–C couplings. Due to the presence of highly complex polymeric mixtures of structural and conformational isomers of phlorotannins and the absence of available commercial phlorotannin standards, accurate chemical identification from extracts using MS/MS is difficult. Therefore, the optimal approach for identification of specific phlorotannins includes both NMR (1D and 2D) analysis coupled with HRMS. Herein, a library (¹H and ¹³C) of the reported phlorotannins has been generated to assist with further identification. Additionally, a range of phlorotannins have been ascribed bioactivity and potential use as nutraceuticals and supplements, including trials of phlorotannin-rich extracts used as nutritional supplements in livestock feed to improve overall growth and condition. Bioactivity studies have identified neuroprotective, antidiabetic, anticancer, antioxidant, anti-inflammatory, antimicrobial and microbiome-beneficial properties, highlighting a multi-faceted potential of phlorotannins. Overall, the majority of such findings have been generated via biochemical and cell-based assays, with only a limited number of in vivo animal experiments conducted. Further preclinical and clinical studies will be required to comprehensively investigate bioavailability, efficacy and safety of phlorotannins, to further define the potential of these unique brown algal polyphenols in animal and human health.