Use of high-strength steel is one of the factors for rationalizing designs of steel bridges. However, high-strength steel can incur problems with weldability, such as the occurrence of weld cracks. One approach to mitigating such problems is to employ under-matched welded joints; however, the tensile strength of transverse butt-welded joints is uncertain. In this study, tensile tests and elasto-plastic analyses were performed on these joints made of three types of steel with different tensile strengths and soft weld metals. On the basis of the experimental and analytical results, we propose an expression for obtaining tensile strengths of transverse butt-welded joints.