Additive manufacturing is increasingly used for producing a variety of components. These components are often not self-contained, and therefore to form a working assembly, they must be attached to other components. In this study, ABS and PLA, two commonly used 3D printing materials, were 3D printed utilizing a Fused Deposition Modeling (FDM) machine. The 3D-printed parts were then rotary friction-welded to solid (non-printed) ABS parts. Rotating-beam fatigue experiments at various stress levels were used to evaluate the fatigue strength of these weld joints. Fatigue testing was also carried out on solid ABS/solid ABS and non-welded ABS samples. Furthermore, a comprehensive analysis of the fracture surfaces was performed employing field emission scanning electron microscopy (FESEM). According to the results, non-welded ABS, solid ABS/solid ABS, solid ABS/3D-printed ABS, and solid ABS/3D-printed PLA had the best fatigue performance in this order.