Incorporation of unused agricultural by-products into materials is a relevant strategy in developing biosourced and economically competitive products that limits the environmental impacts of plastics. Development of 3D printing techniques offers the possibility to design such biomaterials while bringing new functionalities, however, it is critical to characterize and control both the plant material properties and the interactions between the plant material and the polymeric matrix during the whole process, from filament production to 3D printing. In this study, flax shives were selectively milled and then used as a starting material to be grafted to a fluorophore whose fluorescence varies under pH. The resulting fluorescent shives were processed with poly-(butylene-terephthalate) (PBAT) by extrusion to produce a filament reinforced with 10 %-wt of flax shives, which was the subsequently 3D printed. Extensive microstructural characterization (particle size and shape analysis, X-ray microtomography) demonstrated that the flax particles were homogeneously distributed into the 3D printed material. Despite the relatively low content of fluorescent flax shives in the final 3D printed material (1%-wt) and successive heating stages (during extrusion and 3D printing), a strong fluorescent emission could still be measured. This work paves the way for using fluorescent flax shives as reinforcements into composites, thus making 4D materials with potential applications as sensors depending on the fluorophore used. 50 μm). In particular, a greater reactivity can be obtained through