For high structural performance and constructability, a steel–concrete composite wall with boundary elements of steel U-section was developed. In the present study, cyclic lateral loading tests were performed to investigate the flexural performance of the proposed walls. Test parameters were the type of boundary element, area of steel U-section, and type of web reinforcement. The boundary steel U-section not only provided high flexural strength, but also provided lateral confinement to the boundary concrete and restrained shear sliding of the web concrete. Thus, the flexural strength, deformation capacity, and energy dissipation were significantly greater than those of the counterpart RC wall with conventional boundary reinforcement. Such advantages were pronounced when steel U-section with larger area was used. Further, the use of steel plate beams restrained diagonal cracking and spalling of the web concrete, thus improving the deformation capacity. As strain hardening stress after yielding was developed in the boundary steel section, the strength of the test specimens exceeded the nominal flexural strength based on plastic stress distribution.