Demand for dissimilar steel/aluminum-alloy joints for use in lightweight multi-material automobile bodies has increased. In this research, a high-speed brazing technology was developed by combining the hot-wire method with a high-power diode laser. The hot-wire laser brazing conditions required to achieve high brazing speed were optimized using high-speed imaging and cross-sectional observations. Joint properties, such as intermetallic compound formation and tensile strength, were evaluated. Finite-element analyses were performed on tensile test specimens of different joint geometries to investigate this effect on joint strength. The high-speed brazing condition achieved by the combination of the hot-wire and high-power diode laser significantly suppressed intermetallic compound formation. Sound joints with high tensile strength were obtained at a high brazing speed of up to 9 m/min. Finite-element analysis results showed that a sufficient joint length compared with the base metal thickness was important to achieve high strength and larger elongation.