Okadaic acid is known as a diarrheal shellfish poison. It is thought that there is no specific target organ for okadaic acid after it has been absorbed into the body. However, the details of its pharmacokinetics are still unknown. In this study, we demonstrated that okadaic acid was more toxic to the hepatocyte-specific uptake transporter OATP1B1- or OATP1B3-expressing cells than control vector-transfected cells. In addition, PP2A activity, which is a target molecule of okadaic acid, was more potently inhibited by okadaic acid in OATP1B1- or OATP1B3-expressing cells compared with control vector-transfected cells. The cytotoxicity of okadaic acid in OATP1B1- or OATP1B3-expressing cells was attenuated by known substrates of OATP1B1- and OATP1B3, but not in control vector-transfected cells. Furthermore, after uptake inhibition study using OATP1B3-expressing cells, Dixon plot showed that okadaic acid inhibited the uptake of hepatotoxin microcystin-LR, which is a substrate for OATP1B1 and OATP1B3, in a competitive manner. These results strongly suggested that okadaic acid is a substrate for OATP1B3 and probably for OATP1B1, and could be involved in unknown caused liver failure and liver cancer. Since okadaic acid possesses cytotoxicity and cell proliferative activity by virtue of its known phosphatase inhibition activity.