This paper summarizes an experimental study of steel pipes repaired with patch plates using underwater wet welding to evaluate strength and ductility. In this study, the thickness of a portion of steel pipes was reduced by half to simulate corrosion damage, and steel patch plates were fillet-welded to cover the reduced thickness area. For one set of specimens patch plates were welded in the open air, and for the other they were welded in the underwater wet environment to understand effects of welding environments on strength and ductility of repaired steel pipes. The specimens were tested in flexure or compression until failure. Based on the experimental results, recoveries of stiffness, strength, and ductility were evaluated, and the effectiveness of patch plates was also examined.