Strong, thermally insulating and biological PLA foams derived from cost-effective and environmentally friendly process are needed for saving resources and improving energy efficiency. PLA’s brittle feature and poor foaming ability are currently the major challenges which strongly restrict developing high-performance PLA foams. Herein, a scalable, cost-effective and environmentally friendly route by combining in-situ nanofibril reinforcement with high-pressure microcellular injection molding (HPMIM) was developed for producing high-performance PLA foams. Nanofibrillar PLA/PET composite was prepared by coupling twin-screw compounding and melt spinning. The presence of PET nanofibrils with an average diameter of 114.8 nm greatly improve PLA’s crystallization, viscoelasticity and melt strength, thereby enhancing its foaming ability. Thus, sub-microcellular nanofibrillar PLA/PET composite foam with an average cell size of 322.8 nm was produced using HPMIM, which shows dramatically enhanced mechanical properties than the pure PLA foam. Remarkably, the Izod and Gardner impact strengths of the HPMIM-fabricated PLA/PET foam are respectively more than 7-fold and 6-fold higher than those of the PLA foam fabricated by regular microcellular injection molding. Moreover, nanofibrillar PLA/PET composite foams with expansion ratio varying from 3.8-fold to 26.2-fold were fabricated. Thanks to the unique nanostructured cell walls with numerous micro/nano fibrils and voids, these nanofibrillar PLA/PET composite foams with large expansion ratios show outstanding thermally insulating properties. The 26.2-fold PLA/PET composite foam exhibits a thermal conductivity of 26.8 mW/m·K, much better than that of current polymer foams. The scalable and environmentally friendly method combined with the unique foam structure opens up new perspectives for developing high-performance multifunctional porous materials.