Additive manufacturing provides a significant alternative for producing complex geometries, enabling rapid prototyping and manufacturing of polymer-based components. However, the build volume limitations of largescale 3D-printed parts make joining of components inevitable. Friction Stir Spot Welding (FSSW) is an effective method for joining polymers and composites with low weldability, but its mechanical stirring capability may be limited for some polymer systems. Ultrasonic-assisted friction stir spot welding (UA-FSSW) enhances material softening and promotes homogeneous material flow, improving joint quality. This study investigates the join-ability of 3D-printed polylactic acid (PLA Plus) and carbon fiber-reinforced PLA (PLA-CF) composites using UA-FSSW. Experiments were conducted at a fixed rotational speed (1200 rpm), plunge rate (10 mm/min), stirring time (10 s), and ultrasonic frequency (20 kHz), while five amplitude ratios (0 %, 30 %, 50 %, 70 %, 90 %, 100 %) were applied to evaluate the effect of ultrasonic vibration. Lap-shear tests and microstructural analyses were performed to assess weld quality and the relationship between process parameters and joint performance. All welded joints exhibited higher tensile-shear strength compared to reference materials. The highest joint quality was achieved with PLA Plus on top and PLA-CF on the bottom at 90 % amplitude, reaching welding efficiencies of 57.2 % for PLA Plus and 154.4 % for PLA-CF. This configuration improved material flow, minimized interfacial defects, and promoted formation of a more homogeneous weld region. The findings demonstrate the effectiveness of UA-FSSW in enhancing the mechanical performance of polymer-based 3D-printed assemblies and provide guidance for future applications in additive manufacturing.