This study focused on the electrochemical corrosion behavior of diffusion welded CoCrNi medium entropy alloy (MEA) and SUS 304 stainless steel joints. The MEA and SUS 304 alloys joined through a vacuum diffusion bonding process at three different bonding temperatures (950, 1000, and 1050°C) at 5 MPa compressive pressures with an hour holding time. The electrochemical corrosion behavior of the welded alloys was analyzed via potentiodynamic polarization and electrochemical impedance spectroscopy scans. All the tests were performed in simulated seawater solution (3.5% NaCl) at room temperature in steady-state conditions. The characteristic electrochemical results of the diffusion welded joints at 1000°C showed an excellent corrosion property due to its lowest corrosion current density Icorr, highest pitting potential Ep value, a large stable passive region, highest polarization resistance Rp, and the existence of double-layer impedance. The presence of voids and the formation of various intermetallic phases at the grain boundaries and aggressive Cl ion activity lead to the reduction in the corrosion resistance of the samples bonded at 950 and 1050°C.