The development of renewable biomass-based bioplastics is one solution to reduce environmental pollution caused by the use of conventional petroleum-based plastics. One potential source of biomass is Ulva lactuca, which contains ulvan polysaccharides that can be used as a material for forming biodegradable bioplastic films. This study aims to determine the optimum conditions for the synthesis of ulvan-based bioplastics using the Response Surface Methodology (RSM). Bioplastics were synthesized using the solution casting method with ulvan filtrate as the main matrix and polyvinyl alcohol (PVA) as the supporting polymer, with glycerol and sorbitol as plasticizers and citric acid and boric acid as cross-linkers. Optimization was performed using a Box–Behnken design with 17 experiments, with independent variables of boric acid mass (0.025 - 0.075 g), mixing temperature (65 - 75 ˚C), and mixing time (15 - 25 minutes). The optimization results showed that optimal conditions at a mixing temperature of 70 °C, boric acid mass of 0.05 g, and mixing time of 20 minutes, with point prediction of 4.1476 MPa in tensile strength, thickness of 0.088 mm, and desirability value of 1. The statistical model for tensile strength demonstrated an R2 value of 0.9468, an adjusted-R2 of 0.8785, and a lack-of-fit value of 0.3024. Meanwhile, the thickness model exhibited an R2 value of 0.9792, an adjusted-R2 of 0.9524, and a lack-of-fit value of 0.7510.