Since 2012 the Moroccan Atlantic coast has witnessed the introduction of the brown alien seaweed Sargassum muticum with a high invasive potential presenting environmental and economic risks. This study evaluates the effect of seasons and location on the biochemical composition of S. muticum. Proteins, carbohydrates, lipids, pigments, minerals, and heavy metals were studied during four seasons from spring 2019 to autumn 2020 at four sites along the Moroccan Atlantic coast: El Jadida, Saada (SA), Sidi Bouzid (SB), Moulay Abdallah (MA) and Jorf Lasfar (JL). MA presents the highest protein content (12.5% dw) during spring and the highest level of lipid and carbohydrate content during summer (1.8% dw and 41.5% dw respectively). The highest chlorophyll a content was found in JL and MA sites with (21.6 and 21.45 mg mL$^{-1}$, respectively) during winter. Maximum chlorophyll c and carotenoids were found at SA with (2.27 and 9.71 mg mL$^{-1}$, respectively) during winter. In all locations and during all seasons, the mineral profile was rich in essential minerals such as magnesium, manganese, zinc, iron, selenium, and lithium. Levels of toxic metals Al and Pb were lower than in previous studies whereas Cd and As were slightly higher than the legislation limits for dry seaweeds. Therefore, in this context, some strategies of S. muticum valorisation should be favoured such as bioenergy, bioplastic or biostimulant over others such as human food and animal feed.