The present study aims to investigate the economic and process feasibility of multiple biorefinery pathways based on a natural and widely available brown macroalga, Nizimuddinia zanardini. In the first pathway, named “biofuel scenario”, the seaweed is used for large-scale energy and fuel production by converting all fermentable ingredients, including mannitol and glucan, to bioethanol, and employing the waste streams to produce electricity. In the second pathway, named “biorefinery scenario”, a full cascading biorefinery approach was applied to co-produce highly valuable chemicals, including alginate, mannitol, and protein. Moreover, bioethanol and fertilizer were produced from the remaining organics and inorganics, respectively. Both scenarios were simulated by Aspen Plus® and the simulation results were used in a cash flow analysis, by which multiple economic indices were calculated. In the biofuel scenario, bioethanol and electricity were produced at the rate of 35,000 ton/year and 26 MW, respectively. While in the biorefinery scenario, 59,000 ton sodium alginate, 40,000 ton mannitol, 47,000 ton protein, 120,000 ton fertilizer, and 19,000 ton bioethanol were produced annually. Moreover, in the biofuel scenario, hot utility, cold utility and electricity were utilized at the rate of 29.8 MW, 76.6 MW, and 10.3 MW, respectively. While in the biorefinery scenario, 69.5 MW hot utility, 57.6 MW cold utility, and 59.9 MW electricity were demanded. In addition, in the biofuel scenario, total capital investment (TCI), total operating costs (TOC), and total annual sales (TAS) were 261.7 million US dollar (M$), 57.3 M$/year, and 24.9 M$/year, respectively. On the other hand, in the biorefinery scenario, higher values were obtained for the TCI (522.5 M$), TOC (100.2 M$/year), and TAS (315 M$/year). The maximum allowable dry seaweed price was calculated for the biorefinery scenario as 374 $ per tonne of dry weight, which could be used to set a seaweed cultivation cost goal in the future studies. Minimum ethanol selling price (MESP) was 2.5 $ per liter for the biofuel scenario, which was significantly higher than MESP reported for well-known lignocellulosic biomass resources, showing that third-generation biofuel from seaweed had a lower economic potential compared to the second-generation biofuel. In the biofuel scenario, the major MDSP contributers were capital recovery charge and ethanol product credit. While the main contributers to MDSP of the biorefinery scenario were alginate product and mannitol product credits, showing the importance of producing high-value chemicals in the seaweed biorefinery. Finally a sensitivity analysis of multiple process and economic parameters was applied on the MDSP of each scenario. In the biofuel scenario, MDSP altered significantly by changing the bioethanol production rate, illustrating that the ethanol production rate played a pivotal role in the economics of a plant producing a large volume of biofuel from seaweed. While in the biorefinery scenario, changes in alginate composition in the feedstock can considerably affect the MDSP, and in turn, the project's economics. Among different economic parameters, changes in the percentage of initial investment, and the internal rate of return, substantailly altered MDSP of both scenarios. Based on the profitability indices, it was concluded that the seaweed had a low economic potential for large-scale biofuel production owing to the low ethanol production yield from seaweed carbohydrates. To outcome this obstacle, studies should be performed on commercializing microorganisms capable of producing ethanol at a high rate, especially from those carbohydrates that are exclusively available in the seaweed structure. On the other hand, a cascading biorefinery based on seaweed feedstock showed promising economic performance, producing large volumes of high-value biobased chemicals.Therefore, the ever-increasing human demand for chemical products can be addressed sustainably by establishing cascade biorefineries based on seaweed feedstock, which is the largest unexploited renewable carbon resource on the planet.