Six diets with different concentrations of L-phenylalanine (10.71, 12.71, 14.71, 16.71, 18.71, and 20.71 g kg^–1 dry diet) were offered to the Pacific white shrimp Litopenaeus vannamei (Boone) reared in low-salinity water (0.50–1.20 g L^–1) to determine the phenylalanine requirement. After an 8-wk feeding trial, the results indicated that weight gain was highest in shrimps treated with 16.71 g kg^–1 dietary phenylalanine (diet P4) and that lower or higher dietary phenylanine supplementary reduced weight gain (P < 0.05). Moreover, the highest final body weight, body protein deposition (BPD), and relative muscle weight, and the lowest hemolymph urea nitrogen concentration, alanine aminotransferase, and aspartate aminotransferase activities were also found in group P4 (P < 0.05). Hemolymph metabolite profile showed that deficiencies or excess amounts of dietary phenylalanine mainly affected amino acids and energy metabolism. Polynomial regression calculated using weight gain rate, feed conversion ratio, and BPD indicated that the optimal dietary phenylalanine requirement for L. vannamei reared in low-salinity water was 15.81 g kg^–1 of dry diet, corresponding to 38.56 g kg^–1 of dietary protein.