The effects of integrating different densities of white leg shrimp Litopenaeus vannamei postlarvae (PL) and red seaweed Gracilaria tenuistipitata on water quality and shrimp performance were assessed in the nursery phase. A 3 × 4 factorial experiment with three levels of shrimp density (1000, 2000, and 3000 PL m⁻³) and four levels of red seaweed density (0, 1.0, 1.5, and 2.0 kg m⁻³) was randomly designed in triplicate. Shrimp PL12 (mean weight of 4.27 ± 0.28 mg) and red seaweed were stocked in 150-L tanks at a salinity of 15 g L⁻¹ for 30 days. The results indicated that the integration of shrimp PL and red seaweed significantly reduced the nitrogen and phosphorus contents in the culture tanks and improved the survival and growth rate of shrimp, although not significantly. A significant interaction effect (p < 0.01) between shrimp and seaweed densities was only observed for shrimp production. Notably, higher stocking density resulted in lower growth performance but enhanced production. Applying stocking densities of 1000 and 2000 PL m⁻³ reared shrimp that were bigger in size, while a density of 3000 PL m⁻³ obtained the highest production output in the integrated system. Following the 30-day growth trial, shrimp quality was examined through a bacterial challenge test using pathogenic Vibrio parahaemolyticus on the shrimp groups previously reared at a density of 3000 PL m⁻³ and integrated with different amounts of seaweed. After 14 days of immersion challenge, the cumulative mortality in the control group was significantly higher than that of other groups. This result suggests that the presence of G. tenuistipitata improved the antibacterial activity of L. vannamei against V. parahaemolyticus during the nursery phase.