This study focused on the performance evaluation of clay and thermomechanical pulp (TMP) fiber to reinforce low- and high-density polyethylene (LDPE and HDPE) biocomposites. A 23 factorial experiment was designed using two levels of clay, TMP fibers, and PE as variables. Mechanical properties, thermal behavior, melt flow index, and water absorption were evaluated. In HDPE, the partial replacement of TMP fiber with 10 wt% clay increased the melting point. Clay also reduced the main polymer degradation temperature in both matrices (LDPE and HDPE). The mechanical properties of the samples with 20 wt% fiber and 10 wt% clay were similar to or better than those containing 30 wt% TMP, that is, tensile strength and modulus of 34 and 2700 MPa, compared to 30 and 2400 MPa, respectively. Although the water absorption increased with the addition of TMP fiber and clay, the water absorption of the composite with 20 wt% TMP and 10 wt% clay was relatively low and similar to the biocomposite containing 30 wt% TMP, that is, 1.15 and 1.07% after 30 days, respectively. The comparable properties of biocomposites with 30 wt% TMP and biocomposites with 20 wt% TMP and 10 wt% clay demonstrate the potential of clay to reduce the cost of the final product. Highlights: Clay enhances the tensile modulus and strength, and reduces the color darkening, compared to TMP. TMP fibers and clay reduce the melt flow index, elongation, and impact toughness. TMP fibers and clay increase the melting point and reduce the degradation temperature. Reduction in production costs of biocomposites by adding inorganic clay filler.