This study presents a sustainable desert sand–seawater 3D printed concrete (SDS3DPC) designed for infrastructure construction in arid coastal regions, where freshwater scarcity and depletion of river sand pose major challenges. The combined effects of seawater, desert sand, and sand-to-binder (S/B) ratio on printability, mechanical properties, hydration behavior, pore structure, and environmental impacts were systematically investigated and compared with conventional river sand concrete. Experimental results demonstrated that, at a target flow diameter of 130–140 mm, desert sand improved extrudability, while seawater enhanced buildability. The 3D printed desert sand mixtures achieved a compressive strength of 71.46 MPa, exceeding that of river sand mixtures. Increasing the S/B ratio from 0.9 to 1.5 reduced the compressive and flexural strengths by 28.48% and 49.58% at 28 d, respectively. Microstructural analyses revealed that the synergistic incorporation of seawater and desert sand accelerated early hydration and promoted the formation of a denser C-S-H gel network. Moreover, SDS3DPC exhibited refined pore structures characterized by lower capillary absorption, higher tortuosity, and increased micro-fractal dimensions relative to river sand mixtures. A region-specific life cycle assessment further indicated that SDS3DPC could reduce carbon emissions and total costs by 55.9% and 69.4%, respectively. These findings highlight the significant potential of SDS3DPC as a low-carbon and resource-efficient construction material for additive manufacturing in arid coastal environments.