The pursuit of extended driving range and enhanced energy efficiency in electric vehicles (EVs) necessitates the systematic reduction of mass in all non-rotating auxiliary components, including the reduction gearbox housing. This paper presents a comprehensive methodology for the lightweight design and structural topology optimization of a single-stage EV reduction gearbox housing. The primary objective is to achieve a significant reduction in mass while maintaining or improving upon the original design’s structural performance under critical load cases, including static stiffness, dynamic vibrational characteristics, and fatigue life. The process begins with the establishment of a baseline finite element model derived from a conventional housing design. Operational load cases are defined based on maximum torque transmission, emergency braking, and mounting point excitations. A multi-stage topology optimization procedure is then implemented, employing a density-based method to generate a conceptual material layout that maximizes static stiffness per unit mass. The optimized topology is subsequently interpreted into a smooth, manufacturable geometry, followed by meticulous parametric size and shape optimization of the resulting rib network and wall thicknesses. Detailed static, modal, and harmonic response analyses are conducted on the final optimized design. The results demonstrate a successful mass reduction of 34.2% compared to the baseline housing. Crucially, this is accompanied by a 12.7% increase in overall torsional stiffness, a 15.3% elevation in the first-order natural frequency, and a marked reduction in vibration response amplitude within the operational frequency range. The study validates the efficacy of integrating topology optimization with detailed follow-on design and analysis, providing a robust framework for developing lightweight, high-performance gearbox housings that contribute directly to improved EV efficiency.