Oilfield wastewater (OFW) is a by-product of petroleum production and has a high nitrogen concentration. Bioremediation by macroalgae appears to be an option for OFW, and the genus Ulva (Chlorophyta) has strong potential as an agent of bioremediation because of its high nitrogen absorption capacity. This experimental study evaluated the efficiency of bioremediation of nitrogenous compounds in three concentrations of OFW by Ulva lactuca. One-phase decay models, photosynthetic status (assessed by pulse amplitude–modulated [PAM] fluorometry), and growth rate were used to assess bioremediation efficiency and algal physiology. All nitrogenous compounds were removed during the experimental period. The models that were applied for ammonium showed a stronger bioremediation effect for OFW at a concentration of 25% than at concentrations of 12.5% and 2.5%, and for nitrate, the models showed a stronger bioremediation effect at the 12.5% OFW concentration. The minor effects of OFW on photosynthetic performance and growth, added to high removal of nitrogen, emphasize the bioremediation capacity of U. lactuca, suggesting a new possibility of bioremediation of this waste.