The main objective of this study was to evaluate the potential adaptation to seawater of the freshwater strain Scenedesmus almeriensis and to evaluate the effect of salinity on its growth, morphology, and biochemical composition. Incorporating low seawater concentrations into the culture medium (up to 103 mM NaCl) resulted in an increase in biomass productivity from 0.15 to 0.22 g·L-¹ ·day-¹ and an increase in the maximum specific growth rate from 0.14 to 0.26 day-¹. This was attributed to the presence of micronutrients in the seawater and adaptive responses to stress. Despite a lower biomass productivity (0.11 g·L−1·day-¹), Scenedesmus almeriensis was able to grow well (0.15 day−1) in a medium formulated with only seawater and commercial fertilisers. Cell morphology was significantly affected, with a 150 % increase in cell perimeter and an increase in roundness from 61.5 % (freshwater) to 95.8 % (seawater). The use of seawater also affected the chemical composition of the biomass. Seawater favoured the synthesis of specific fatty acids that have nutritional and industrial value, including polyunsaturated fatty acids. The protein content was slightly reduced under saline conditions but remained at 40 % (which is high compared to other biomasses). The results highlight the potential of seawater as a sustainable and cost-effective substitute for freshwater to produce Scenedesmus almeriensis. Future studies will validate the production of these strains at the large scale and identify potential industrial uses for the biomass produced.