<p>Nickel–aluminum bronze (NAB) is increasingly used in marine applications, where components are subjected to combined cyclic loading and corrosive environments. This study investigates the very high-cycle fatigue (VHCF) response of wire-arc additive manufacturing (WAAM) NAB under in-situ synthetic seawater conditions employing high-throughput ultrasonic fatigue (USF) testing. Specimens were extracted from a wall fabricated using optimized WAAM parameters, which was subsequently annealed to reduce residual stresses and stabilize the microstructure. In-situ ultrasonic corrosion–fatigue tests were conducted by immersing the specimens in a synthetic corrosive medium during cyclic loading. Results show a significant reduction in fatigue life under in-situ corrosion compared to ambient air (i.e., non-corroded specimens), with all cracks initiating at the specimens’ surface. Corrosion–fatigue–induced pits, along with the surrounding corrosion-induced discolored zone (CDZ), were consistently observed across the fracture surfaces of all specimens, whereas no evidence of internal WAAM-induced volumetric defect–driven failure was identified. The size of CDZ increased with decreasing stress amplitude, reflecting cumulative damage accumulation. These findings demonstrate that surface corrosion governs VHCF failure in WAAM NAB and highlight the importance of considering corrosion–fatigue interaction (coupling) in the design, surface protection, and life assessment of marine components.</p>