This work evaluated the use of effluent from a marine shrimp biofloc rearing system to cultivate the green seaweed <em>Ulva</em>. First, the growth of two <em>Ulva </em>species, <em>U. ohnoi</em> and <em>U. fasciata,</em> was evaluated. Second, the best-performing species was cultivated under two different stocking densities (2 g L^-1 and 4 g L^-1) to evaluate both growth and nutrient uptake rates, considering total ammonia nitrogen, nitrate, and orthophosphate. In both cases, environmental variables were monitored, and the cultivation medium, consisting of 25% biofloc water and 75% seawater, was exchanged weekly. <em>U. ohnoi</em> grew significantly better, considering all variables evaluated (<em>p</em>&lt;0.05). The smaller stocking density produced a higher specific growth rate (<em>p</em>&lt;0.05). Yield, however, was unaffected (<em>p</em>≥0.05). No significant differences in the nutrient uptake rates were observed (<em>p</em>≥0.05). Overall, this work highlights the importance of species selection for seaweed destined for aquaculture. Additionally, it also optimizes the cultivation of seaweeds, specifically <em>U. ohnoi</em>, using effluent from biofloc systems.