Poly(lactic acid) (PLA) is a biopolymer that resembles oil-based plastics in its mechanical properties. However, in manufacturing techniques such as fused deposition modeling (FDM), the inherent characteristics of the process led to a reduction in the mechanical properties of PLA-based pieces. To improve the material performance, one strategy is photo-cross-linking PLA chains. Functionalization of PLA requires grafting a molecule capable of reacting with it and ultimately cross-linking the polymeric chains. In this research, methacrylate coumarin was incorporated into PLA via free-radical grafting under reactive extrusion conditions for 5 min. The highest grafting degree value was 0.7% after the addition of 10 and 5 wt % coumarin and dicumylperoxide, respectively. Thereafter, a thin film of the material was ultraviolet (UV)-exposed for 48 h, and photo-cross-linking was confirmed with a gel formation of 16.9%. As a result, the thermal stability and melt strength of pure PLA increased by 7 °C and 1 order of magnitude, respectively. Finally, the tensile properties were evaluated by printing 3D specimens. Photo-cross-linked films were blended with nonirradiated coumarin-based PLA to obtain high infill density values (89% ± 0.8). The mechanical strength (17.8%) and stiffness (26%) of the neat PLA were enhanced