We have characterized a novel Sac domain-containing inositol phosphatase, hSac2. It was ubiquitously expressed but especially abundant in the brain, heart, skeletal muscle, and kidney. Unlike other Sac domain-containing proteins, hSac2 protein exhibited 5-phosphatase activity specific for phosphatidylinositol 4,5-bisphosphate and phosphatidylinositol 3,4,5-trisphosphate. This is the first time that the Sac domain has been reported to possess 5-phosphatase activity. Its 5-phosphatase activity for phosphatidylinositol 4,5-bisphosphate (K(m) = 14.3 microm) was comparable with those of Type II 5-phosphatases. These results imply that hSac2 functions as an inositol polyphosphate 5-phosphatase.