Previously, we have shown that p55PIK, an isoform of class I_A phosphoinositide 3-kinase (PI3K), specifically interacts with important cell-cycle regulators, such as retinoblastoma (Rb), to promote cell-cycle progression. Here, we used the glutathione S-transferase pull-down assay to identify other p55PIK-interacting proteins besides Rb in a Rb-deficient cell line and found that p55PIK interacted with proliferation cell nuclear antigen (PCNA), which plays a key role in coordinating both initiation of the leading strand DNA replication and discontinuous lagging strand synthesis. Overexpression of p55PIK increased, and knockdown decreased, DNA synthesis and DNA replication by modulating the binding of DNA polymerase δ (Polδ) to PCNA. Moreover, a cell-permeable peptide containing the N-terminal–binding domain of p55PIK (TAT–N24) disrupted the p55PIK–PCNA interaction in cancer cells, and also inhibited the DNA synthesis and tumor growth in cell culture and in vivo. Altogether, our results show that the p55PIK–PCNA interaction is important in regulating DNA synthesis and contributes to tumorigenesis. Furthermore, the p55PIK–PCNA interaction provides a potential new target for anticancer drug development. Mol Cancer Ther; 12(10); 2100–9. ©2013 AACR.