Alkali-treated coir fibers were modified by silane coupling agent in a microwave oven. The use of microwave-assisted chemical treatments efficiently promoted the esterification reaction to improve the interfacial adhesion between the coir fibers and PLA matrix. Effects of the treated coir fiber content (1 wt.% to 7 wt.%) on the surface morphology and tensile, impact, and thermal properties of PLA/coir fiber biocomposites (AKWCF/PLAs) were evaluated. At a coir fiber content of 1%, the AKWCF/PLAs showed a remarkable increase of 28% in the percentage impact strength, while the tensile strength and breaking strength decreased with increasing coir fibre content. The thermal stability of the AKWCF/PLAs worsened and the degree of crystallinity increased with increasing fiber content. The decreased cold crystallization temperatures of AKWCF/PLAs further confirmed the role of coir fibers treated with the new combined method as an effective nucleating agent.