Lightweight plastic materials are important for saving resources and energy, reducing environmental pollution, and achieving sustainable development. Foam injection molding is a promising technology for manufacturing lightweight plastic components. However, these plastic components present poor mechanical properties and imperfect surface appearances. Herein, we reported a novel strategy to prepare lightweight and tough polypropylene (PP)/polytetrafluoroethylene (PTFE) nanocomposite parts with defect-free surfaces by combining in situ fibrillation and nanocellular injection molding technologies. The nano-fibrillary PP/PTFE nanocomposite was firstly prepared using an in situ method based on twin-screw compounding. Scanning electron microscopy (SEM), rheological and differential scanning calorimetry (DSC) analysis, combined with online optical microscopy observation, demonstrated the network structure of PTFE nanofibrils and its positive effects on melt strength and promoting crystallization. Using nanofibrillary nanocomposites, we achieved nanocellular foaming, for the first time, using the foam injection molding process. The nanocellular PP/PTFE nanocomposite foam thus obtained significantly enhanced mechanical properties compared to the regular PP foam, and even superior strength and ductility compared to unfoamed PP. In particular, the impact strength of the nanocellular foam was 700% higher than that of the regular foam and 200% higher than that of the unfoamed product. Moreover, unlike regular foam, the nanocellular PP/PTFE nanocomposite foam showed outstanding surface appearance without any silver or swirl marks. More importantly, the whole process was facile, flexible, efficient, and easy to scale-up, and could be easily extended to other materials. The remarkable mechanical performance and surface appearance, combined with the flexible and extendable process, confers nanocellular PP/PTFE nanocomposite foams a promising future in many advanced applications where both lightweight and mechanical integrity are required.