It is now clear that CD8+ T cells are crucial for therapeutic immunity against chronic viral infections and/or tumors. We reason that a strategy capable of improving CD8+ T cell activation would improve the efficacy of protein-based vaccines, which predominantly generate CD4+ T cell-mediated responses. Herein, we explore the ability of a novel cell-penetrating peptide (CPP), LAH4, to facilitate intracellular delivery of protein-based vaccines adjuvanted with Toll-like receptor 9 agonist CpG oligonucleotide (CpG) to generate enhanced CD8+ T cell immune responses and antitumor effects. LAH4 was found to mediate the intracellular delivery of both protein and nucleotide cargo and facilitate protein internalization using mechanisms involving endosomal acidification and processing through the proteasome pathway, leading to enhanced cross presentation of protein antigen by dendritic cells to CD8+ T cells. LAH4 also improved the internalization of CpG, resulting in NFkB activation, thus potentiating the adjuvant effect of CpG. We found that protein-based vaccine comprised of LAH4 mixed with model antigen and CpG generated significantly improved antigen-specific CD8+ T cell immune responses and/or antitumor effects. Furthermore, we found that LAH4 was able to enhance the ability of a tyrosinase-related protein 2 (TRP-2) peptide-based vaccine to generate TRP2-specific CD8+ T cells and antitumor effects against TRP2-expressing tumors. Thus, our results suggest that CPP technology using LAH4 is able to enhance both protein-based and peptide-based vaccine potency to generate antigen-specific CD8+ T cells and antitumor effects. Our findings serve as an important foundation for future clinical applications of CPP technology to improve protein/peptide-based vaccine potency.