With the advancement of industry, researchers increasingly aim to join different materials to benefit from the superior properties of each component. In this study, we investigate the friction stir spot welding (FSSW) behavior of 3D-printed PLA-based materials, specifically PLA Plus (PLA + ) and short carbon fiber-reinforced PLA (PLA-CF). We welded single-material (PLA + /PLA + , PLA-CF/PLA-CF) and multi-material (50% PLA + /50% PLA-CF) specimens at a plunge rate of 10 mm/min using rotational speeds of 600, 900, and 1200 rpm. We evaluated weld quality through mechanical testing and fracture surface analysis. Our results show that weld efficiency-calculated as the ratio of the maximum lap-shear load of the welded joint to that of the corresponding base material-ranges from 52.3 to 108.7% for single-material joints and from 60.4 to 135.5% for multi-material joints. These findings demonstrate that proper material positioning and optimized welding parameters improve mechanical performance, with multi-material joints outperforming single-material ones.