Abstract: Welding is a core process ensuring the construction and full life-cycle operation of marine equipment and subsea pipelines; however, welded joints are exposed to harsh seawater environments for long periods, and their microstructures and properties directly determine the service safety of marine engineering structures. The effects of process parameters such as current on the microstructural evolution and corrosion behavior of X65 pipeline steel welded joints fabricated by hyperbaric dry gas metal arc welding (GMAW) were investigated. By systematically regulating ambient pressure, welding current, and welding voltage, the microstructural morphology, corrosion uniformity, and macroscopic service performance of the weld zone were comprehensively characterized using metallographic analysis, electrochemical testing, and immersion corrosion tests. The results indicate that the weld microstructures under various process parameters are mainly composed of acicular ferrite and lath ferrite, without changes in the phase transformation type. Elevated heat input and increased ambient pressure optimize structural uniformity or refine microstructures, thereby reducing electrochemical reactivity and enhancing the corrosion resistance of joints. When the ambient pressure increases from 0.1 MPa to 2.0 MPa, there is a decrease of approximately 6.13 × 10⁻⁸ A/cm² in corrosion current density, an increase of 388.79 Ω·cm² in the capacitive loop radius of impedance spectra, a reduction of approximately 62.9% in maximum corrosion depth, and a drop to approximately 0.15 mm/a in the corrosion rate at the end of immersion. In the single-factor tests of current and voltage, the corrosion current densities at 195 A and 28.0 V are 8.473 × 10⁻⁷ A/cm² and 9.384 × 10⁻⁷ A/cm², respectively, both exhibiting superior corrosion resistance.