The production of bioethanol by cyanobacteria can become economically sustainable if other valuable commercial products, such as exopolysaccharides (EPS), are produced simultaneously. This study investigates the feasibility of producing bioethanol and the commercially important EPS from a non-nitrogen fixing marine cyanobacterium, Synechococcus elongatus BDU 10144 in a novel low-cost seawater-fertilizer medium for the development of a biorefinery strategy. Herein, agricultural fertilizer diammonium phosphate (DAP) (nitrogen source), magnesium sulfate (MgSO₄) and potassium were optimized by the central composite design to produce maximum biomass and carbohydrate accretion. The optimal physical conditions for attaining maximum growth and carbohydrate accumulation were determined to be pH 10, 50 μmol m⁻² s⁻¹ (light intensity) and 25°C. Further, different concentrations of seawater were mixed with the optimized fertilizer-salts, and it was observed that 70% of seawater mixed with the optimized fertilizer-salts (hereafter FSW medium) was an optimum condition for growing the test cyanobacterium, which ultimately reduced the cost of the medium by >40 times compared to the standard artificial seawater nutrient (ASN-III) medium. Under the optimal conditions, the maximum biomass (1.79 g/L) and carbohydrate (702.4 mg/L) yield obtained were ~1.7 and 2.2 times higher than the ASN-III. The yield of bioethanol was noted to be 318.1 mg/L, which was ~2.3 times greater than ASN-III. The commercially important EPS was obtained from the discarded supernatant (0.28 g/L), which was ~1.6 times higher in the FSW medium. Thus, the present investigation paves a way forward for mass cultivation of the S. elongatus BDU 1044 in the novel FSW medium for the cost-efficient and sustainable development of a biorefinery concept.