Development of reliable magnesium (Mg) to steel joining methods is one of the critical issues in boarder applications of Mg in automotive body construction. However, due to the large difference of melting temperatures of Mg and steel, fusion welding between two metals is very challenging. Ultrasonic spot welding (USW) has been demonstrated to join Mg to steel without melting and to achieve strong joints. However, galvanic corrosion between Mg and steel is inevitable but not well quantified. In this study, corrosion test of ultrasonic spot welds between 1.6-mm-thick Mg AZ31B-H24 and 0.8-mm-thick galvanized mild steel was conducted. No specific corrosion protection was applied in order to study the worst corrosion behavior. Corrosion test was conducted with an automotive cyclic corrosion test, which includes cyclic exposures of dipping in the salt bath, air drying, then a constant humidity environment. Lap shear strength of the joints decreased linearly with the cycles. No useable joint strength was preserved after about 17 cycles of exposure. X-ray diffraction analysis confirmed the formation of Mg(OH)₂ deposit in the crevice between AZ31 and steel sheets and on the surface of AZ31. The deposit grew thicker with cycles and provided enough force to bend the AZ31 and steel, and caused gradually opening of the joint. Examination of fracture surfaces found that welded areas also decreased with exposure cycle, but even in the weakest joints there was evidence of metal-metal bonding.