This study evaluated the environmental, production, and economic benefits of integrating the seaweed Ulva with the sea urchin Tripneustes gratilla in a commercial-scale recirculating IMTA (integrated multitrophic aquaculture) system. Key benefits include Ulva's biofiltration of urchin farm effluent and its potential as feed for T. gratilla. A farm-scale model was developed to explore these benefits where a system consisting of 42 urchin tanks (8.5 m3 each) and a 300 m2Ulva raceway under different feeding scenarios (fresh Ulva, formulated feed and a mix) was simulated. The model predicted that the urchin system emits an average of 28 g total ammonia nitrogen (TAN) per day. While the Ulva raceway could remove 100% of the TAN emitted, this quantity of TAN is insufficient to sustain Ulva growth leading to a net reduction of Ulva biomass. However, with design or management adjustments the nitrogen provided in the urchin feed, which is lost in the settled solids (sludge), could be utilized through mineralization for Ulva production, creating a highly circular and efficient IMTA system. The model also indicated that ammonia toxicity is unlikely to limit T. gratilla production, with predicted TAN levels below 0.018 mg/L. The system’s projected productivity is substantial, with an estimated annual yield of 323 t WW/ha for whole urchins and 72 t WW/ha for gonads, outperforming other high-value invertebrate species. This productivity suggests that T. gratilla aquaculture could become a viable and profitable industry, with this model offering critical insights for its sustainable development.