This work aimed to evaluate the nutritional composition and immunostimulatory activity of an edible green alga Caulerpa racemosa var peltata collected from South China Sea. The results indicated that this alga contained the total carbohydrates (71.67%, DW), crude protein (11.39%, DW), and crude lipid (1.03%, DW), and was rich in taurine (7.71 mg/100 g, DW), essential amino acids (40.07%), monounsaturated fatty acids (25.72%), and polyunsaturated fatty acids (26.33%). Moreover, water extraction of this alga (WEC) increased NO, ROS, TNF-α, and IL-6 in a dose-dependent manner in RAW264.7 macrophage cells. Furthermore, WEC contained the total sugar (34.19 ± 0.46 mg/g), total protein (6.76 ± 0.32 mg/g), total phenolic (1.93 ± 0.04 mg/g), and total flavonoid (4.80 ± 0.10 mg/g). In addition, the MSⁿ analysis of WEC indicated that the presence of 19 compounds. Thus, these findings suggest that this alga may have a potential nutraceutical value.