Phenolic compounds are found in all brown macroalgae and function as cell wall structure, UV protection and as herbivore deterrents. The concentrations of phenolic compounds vary among taxa and between temperate and tropical ecosystems. Australasia has high concentrations of soluble phenolics compared to other regions. Presently, relationships between phenolic concentrations and environmental gradients are unclear. The purpose of this study was to determine the soluble phenolic concentrations of brown seaweeds along temperate and tropical ecosystems of the Western Australia coastline. We tested the hypothesis that phenolic concentrations are related to local and broad-scale abiotic environmental gradients. Strong environmental gradients of coastal Western Australia provided the opportunity to characterize phenolic compounds across one large gradient. Phenolic concentrations of brown seaweeds at seven study locations varied across latitude with higher concentrations found at higher latitudes and were comparable to seaweeds from similar latitudes in Australia. This trend coincided with a negative relationship between photosynthetically active radiation and phenolic compounds, and a positive relationship with salinity. We also found phenolic concentrations were positively related to salinity in tropical Shark Bay but this was dependent on species. Environmental conditions are important in regulating concentrations of phenolic compounds. Multiple factors influence the concentrations of macroalgal phenolic compounds creating unique distributions among geographical regions. This study highlighted the importance of considering multiple factors when studying phenolic ecology and suggests photosynthetically active radiation and salinity as important drivers of phenolic compound distribution in Western Australia.