Seaweed liquid extracts are an attractive source of phytochemicals with high potential applicability in the aquafeed-additive industry. A commercial seaweed liquid extract (True Algae Max, TAM®), which has a marine seaweed odor, also dislplays significant levels of polysaccharides, phytochemicals, phenolic, and flavonoid compounds showing antioxidant activities and DPPH inhibition. This study investigates the impact of diets supplemented with TAM® as a functional additive on the growth, nutrient utilization, immune responses, and immune-related gene expressions of whiteleg shrimp (Litopenaeus vannamei). A total of 750 postlarvae (PL, with an average initial weight of 0.053±0.001 g) were divided into five experimental groups, comprising three replicates per dietary treatment. For an eight-week experimental period, all groups were fed identical diets except for the variation in TAM® inclusion levels. The basal diet (control diet) had no inclusion level of TAM® (TAM0%). Groups 2–5 each contain TAM® inclusion at levels of 1% (10 mL gk−1 diet TAM1%), 2% (20 mL gk−1 diet TAM2%), 3% (30 mL gk−1 diet TAM3%), and 4% (40 mL gk−1 diet TAM4%), respectively. The results concluded that TAM® has great potential as a feed additive for whiteleg shrimp, compared to the control group. The group TAM2% significantly achieved final weight (4.337 g), weight gain (4.287 g) specific growth rate (3.423% / day), feed conversion ratio (1.970), feed efficiency ratio (0.507), and protein efficiency ratio (1.407), compared to the control group (3.900 g, 3.850 g, 3.347%/day, 2.183, 0.458, and 1.270, respectively). Whole-body composition of protein and lipid contents were significantly improved by all TAM group including TAM2% (51.18% and 4.49%, respectively), compared to the control group (49.38% and 4.15%, respectively). As well as, shrimp in group TAM2% achieved the highest values of lysozyme (3.92 µg mL−1), superoxide dismutase (SOD, 11.92 IU mL−1), catalase (CAT, 13.04 IU g−1), lipase (31.24 IU L−1), and amylase (31.24 IU L−1), compared to the control group (3.19 µg mL−1, 8.80 IU mL−1, 10.84 IU g−1, 17.35 IU L−1, 21.65 IU L−1, respectively). For gene expressions experiment, four immune-related were performed in this study; Peroxiredoxin (Prx), Prophenoloxidase (PPO1), P53-like protein isoform delta (p53), and Hemocyanin subunit L5 (L5H). The results showed that shrimp in group TAM2% achieved the highest significant P53 gene transcription compared to the control group or the other TAM groups. Compared to other groups, the TAM1% group considerably increased PPO1 gene transcription, while TAM3% displayed the greatest Prx gene transcription. On the other hand, compared to control group, all TAM groups significantly improved L5H gene transcription. In conclusion, the current study revealed that TAM® diet supplementation (20 mL of TAM® per kg diet) represents a promising, eco-friendly, and sustainable feed additive in the shrimp aquadiet industry.