Ulva are ubiquitous and opportunistic green algae species that easily adapt to various environmental conditions. These algae are responsible for the green tides that cause many environmental and ecological problems in coastal waters. We investigated the physiological responses of Ulva australis under warming, acidification, and eutrophication conditions. The physiological changes in the algae were observed under various combinations of temperature, pH, and NH4+ levels. Combinations of three temperatures (10°C, 20°C, and 30°C), two pH levels (7.80 and 8.20), and two NH4+ concentrations (4 μM and 120 μM) were considered under laboratory conditions. Temperature, NH4+, and pH had significant impact on the photosynthetic and nutrient uptake rates. However, the 12 h observation could not stimulate the seaweed to change the pH in the cultured media. Changes in relative growth rates, photosynthetic efficiency, and variations in tissue C and N were not affected by the interactions between temperature, pH level, and nutrient concentration. It is probable that, due to global warming, the bloom of Ulva australis may continue in warm, acidic, coastal waters with high nutrient levels.