<p>Ultra-high-performance concrete (UHPC) exhibits favorable bonding performance with normal-strength concrete (NSC) and provides a potential solution for improving the durability of wet-joint interfaces. In this study, uniaxial tensile tests were conducted to investigate the influence of steel fiber content on the bonding performance of UHPC–NSC specimens. Subsequently, the mechanism by which rebar restrains interface crack development in UHPC–NSC specimens was examined through uniaxial tensile tests and four-point bending tests. The results show that the failure characteristics and stress–strain responses of unreinforced UHPC–NSC specimens were essentially independent of steel fiber content. With increasing steel fiber content, the uniaxial tensile strength of UHPC specimens increased approximately linearly, whereas that of unreinforced UHPC–NSC specimens first increased and then decreased, reaching a maximum at a steel fiber content of 2.0%. The bonding performance of the UHPC–NSC interface with 2.0% steel fiber content was superior to that of the NSC–NSC interface. For reinforced UHPC–NSC specimens with 2.0% steel fiber content, two failure modes were observed: rebar debonding failure and UHPC matrix failure. Increasing the rebar diameter and anchorage length, as well as replacing smooth round rebar with ribbed rebar, improved the uniaxial tensile strength of UHPC–NSC specimens and reduced the interface crack width growth rate.</p>