Plant fibres are increasingly used for composite reinforcement, but valorization of by-products such as flax shives still needs to be developed. Also, additive manufacturing, such as Fused Deposition Modelling (FDM), is strongly progressing and can be of great interest to implement biocomposite solutions. The present work focuses on the development of innovative and fully biodegradable filaments for FDM. Three polymer matrices were tested: PLLA, PLLA/PBS 50/50%-wt and PBAT. A filler content of 30-wt% has been achieved using flax shives with PBAT. Flax fibres and shives reinforced 3D printed parts exhibit promising performances and printability, highlighting their potential to successfully develop fully compostable filaments.