A 58-year-old woman was admitted to our hospital for evaluation of left flank pain. Abdominal computed tomography showed a greatly enlarged splenic tumor with a massive portal vein tumor thrombosis (PVTT). We suspected non-Hodgkin lymphoma (NHL) based on the high values of serum soluble interleukin-2 receptor and lactate dehydrogenase. Because there was no superficial lymph node enlargement, ultrasound-guided percutaneous trans-hepatic needle biopsy was performed to obtain a pathological diagnosis of PVTT, instead of a splenectomy, after the patient had provided informed consent. This procedure was thought to be less invasive than splenectomy. Histologic examination revealed CD20-positive NHL. A complete response was achieved after six courses of R-CHOP and it was confirmed by splenectomy. A PVTT due to NHL is extremely rare as compared with that due to hepatocellular carcinoma, gastric cancer, and colon cancer. However, NHL should be considered in the differential diagnosis for a patient with a PVTT, because B cell-NHL tends to have a good prognosis when rituximab combined chemotherapy is administered. We suggest that a percutaneous trans-hepatic needle biopsy may be useful for diagnosing PVTT due to NHL.