In this study, two different surface treatments were evaluated to improve the compatibility of coir fibers with poly(lactic acid). The chemical modifications were achieved by fiber surface treatment with a maleic anhydride grafted to poly(lactic acid) (MAPLA) solution or a poly(methyl vinyl ether‐alt‐maleic anhydride) (MA‐c‐PMVE) aqueous solution. The fibers were analyzed by attenuated total reflectance‐Fourier transform infrared to determine the type and level of surface modification. The biocomposites were compounded via extrusion and molded by injection. Mechanical testing showed increases in tensile modulus (around 16%) and flexural modulus (around 16%) for both treatments, but MA‐c‐PMVE (29%) was more effective than MAPLA (15%) in strength improvement. Finally, the surface treated fibers led to much lower biocomposites water absorption than untreated ones. POLYM. COMPOS., 40:2132–2141, 2019. © 2018 Society of Plastics Engineers