Abstract Biocomposite materials (BMs) containing blends of synthetic polymers and biopolymers are made with the aim of enhancing the best characteristics of each material. In this work, polylactic acid (PLA) and high‐density polyethylene (HDPE) with maleic anhydride‐grafted polyethylene (PE‐g‐MA) blends were reinforced with banana fiber (BF). A simplex lattice mixture design {3,3} was selected to optimize the processing conditions and mechanical properties of the BMs. Fourier transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA) and scanning electron microscopy (SEM) were used to analyze intermolecular interactions, grafting, miscibility, thermal properties and morphology. The optimal BM composition was 37.95 wt% BF and 30/32.05 wt% PLA/HDPE, which exhibited a stabilization time of 2.29 min, a tensile strength of 27.74 MPa and a ductility of 5.26%. The FTIR bands showed that the PE‐g‐MA compatibilizer interacted with the polar BF surface. The BM morphology showed ductile fracture with the absence of BF pull‐outs, consistent with the high tensile strength.