This study proposes a design methodology for achieving weight reduction in joints between metals and plastics, while maintaining joint strength. A metallic insert featuring an Anchor-Lock structure was developed via additive manufacturing and subsequently integrated with plastic through insert injection molding. The geometry of the joint was optimized using structural analysis and topology optimization in ANSYS, and its mechanical performance was evaluated in accordance with KS B ISO 14273. The maximum deformation of the metallic insert was adopted as the evaluation criterion. Ultimately, the optimized geometry achieved up to a 40% reduction in weight compared to the initial geometry, while maintaining joint strength. This study established the validity of a design methodology that simultaneously achieves weight reduction and joint strength in dissimilar material joints.