PTEN (phosphatase and tensin homolog on chromosome 10) is a tumor suppressor whose cellular regulation remains incompletely understood. We identified phosphatidylinositol 3,4,5-trisphosphate RAC exchanger 2a (P-REX2a) as a PTEN-interacting protein. P-REX2a mRNA was more abundant in human cancer cells and significantly increased in tumors with wild-type PTEN that expressed an activated mutant of PIK3CA encoding the p110 subunit of phosphoinositide 3-kinase subunit α (PI3Kα). P-REX2a inhibited PTEN lipid phosphatase activity and stimulated the PI3K pathway only in the presence of PTEN. P-REX2a stimulated cell growth and cooperated with a PIK3CA mutant to promote growth factor–independent proliferation and transformation. Depletion of P-REX2a reduced amounts of phosphorylated AKT and growth in human cell lines with intact PTEN. Thus, P-REX2a is a component of the PI3K pathway that can antagonize PTEN in cancer cells.