Additive manufacturing (AM) is a growing key technology with the potential to make a longstanding impact on the manufacturing sector. Due to its layer-by-layer nature, it allows the production of complex and highly customized components. Currently, different AM techniques are applied for the fast prototyping of devices, components, structures, tools and molds; thus supporting the development process of new products and drastically shortening their commercialization time when compared to traditional methods. As an emerging manufacturing type, AM has a wealth of room to grow in unexpected directions. The present case study aims to explore how this technology can support and improve the joinability of metal-composite hybrid structures produced by Ultrasonic Joining (U-Joining). The process feasibility has been successfully demonstrated to join AM metal and polymer parts. However, further investigation of its main advantages and the joining of subcomponents is still missing. This study aims to demonstrate the application of U-Joining to fabricate AM 316L, and unreinforced and 20% short-carbon-fiber-reinforced poly-ether-ether-ketone (PEEK) hybrid structures. Single lap shear quasi-static performance was assessed in the coupon level, whereby an improvement of 2.7 times in the ultimate lap shear force and 5.9 times in displacement at break was accomplished in comparison to reference specimens. Fracture surface analyses of tested samples exhibited a mixture of cohesive and adhesive failure. Further microstructural analyses at the metal-polymer interface showed micromechanical interlocking between the parts. Finally, a selected skin-stringer-bracket case study was analyzed. It showed a drastic reduction in structural weight by about 64%. To further validate and demonstrate this idea, a scaled-down demonstrator was successfully fabricated.