Given the emerging advances in multi-material additive manufacturing, the joining of dissimilar materials plays an important role in technological and industrial applications. The goal of this investigation is to offer insights on the joining of dissimilar materials produced by different additive manufacturing methods. The investigation involves the joining of 316L stainless steel produced by laser powder bed fusion (LPBF) and polylactic acid (PLA) produced by Fused Filament Fabrication (FFF). The joining of the above-mentioned materials is challenging given the major differences of their melting point, thermal expansion, and shrinkage coefficient. The challenges of filling the interface geometry and shrinkage during solidification of the melt polymer are dominant. New, easy-to-manufacture interface geometries are proposed in variations of the interface, that contained different geometries of cavities and roughness characteristics. The role of roughness, interface geometry, polymer integration in the metallic interface geometry and residual stresses is investigated both experimentally in tension and fatigue and by finite element analysis. A multi-physics thermomechanical transient finite element analysis (FEA) was performed to shed light on the developed thermal stresses at the interface and their impact on the principal stresses developed during tension. Through an appropriate heating of the metallic part and the selection of the adequate cavity geometry and roughness of the interface, an optimized joining of the dissimilar materials can be attained, offering promising strength and endurance values.