Solid-state Li-ion batteries (SSBs) have been regarded as the most promising systems to power electronic devices and electric vehicles because of their high energy density and safety. As the key component of SSBs, solid electrolytes have a substantial influence on battery performance. In recent years, polymeric composite electrolytes (PCEs) that combine polymer electrolytes with various fillers have attracted tremendous attention as a result of their high mechanical flexibility, high interfacial compatibility with electrodes, and easy processability. In this review, we provide a comprehensive summary of the recent progress in filler engineering of polymer electrolytes for SSBs. First, the challenges facing PCEs associated with the physical and chemical strategies for dispersing fillers are discussed. Then, the latest research papers regarding various fillers used in PCEs are classified and reviewed in detail. Moreover, their effects on physical and electrochemical properties, interfacial stability, and Li+-migration mechanisms are discussed, with the aim of improving the understanding of filler-reinforced PCEs for the development of SSBs.