Two biobased polyethylenes (BioPE) and thermomechanical pulp (TMP) fibers were used to produce biocomposites. The impact of TMP fibers on the mechanical properties was assessed in detail. An increase on the viscosity of the melted biocomposites was quantified and was related to the incorporation of the TMP fibers (0–30% w/w). The impact of polyethylene functionalized with maleic anhydride (MAPE) on the mechanical properties was quantified. Compared to neat BioPEs, a maximum increase of tensile strength between 115 and 127% was obtained, for the biocomposites containing 6% w/w of MAPE and 30% w/w TMP fibers. The formulated biocomposites containing 10 and 20% TMP fibers were three-dimensional (3D) printed, by fused deposition modelling. We confirmed that TMP fibers facilitated the 3D printing and correspondingly improved the mechanical properties of the biocomposite materials.