Pig-to-nonhuman primate (NHP) islet transplantation has been widely conducted as a preclinical xenotransplantation model prior to human clinical trial. Portal vein thrombosis is one of the complications associated with islet infusion through the portal vein into the liver. Here, we briefly report severe case of ascites formation accompanied by portal vein thrombi after pig-to-NHP islet xenotransplantation in a rhesus monkey. Meticulous prophylactic treatment such as continuous heparin infusion should be implemented to prevent portal vein thrombi in pig-to-NHP islet transplantation models.