Purpose The aim of this study is to describe our experience with transarterial fiducial marker placement for proton therapy of malignant liver tumors, and to evaluate its technical and clinical efficacy and safety. Materials and Methods Twenty-three patients underwent this procedure. One patient had 2 tumors, and 2 tumors needed 2 markers, so the total number of procedures was 26. The tumors were hepatocellular carcinomas in 16 and liver metastases in 8. Tumor size ranged from 8 to 120 mm (median, 18). A 0.018-inch, 5-mm, straight microcoil (Hilal Embolization Microcoil, Cook, Bloomington, IN) was used as a fiducial marker, and placed at the appropriate position for each tumor. It was decided by consensus of one interventional radiologist and one radiation oncologist. In all cases, it was approached via the right femoral artery with a 4-Fr sheath. Then, a 4-Fr. catheter was placed at the celiac artery. A 1.8-Fr. microcatheter was inserted to the aimed branch of the haptic artery, and then a fiducial coil was placed. Before placement, CT was performed and it was confirmed that the tip of the microcatheter was in an aimed position. We assessed technical success, clinical success, and complication. Technical success was defined as the successful delivery and placement of a fiducial coil, and the clinical success was defined as whether or not proton therapy could be performed without coil migration. Complications associated with the procedure were evaluated with physical examinations and enhanced CT. Results All 26 fiducial coils were placed successfully, so the technical success rate was 100 % (26/26). All 23 patients could undergo proton therapy without coil migration. Thus, clinical success rate was 100% (23/23). A slight bleeding was observed in only one case, due to injury of the small branch during catheterization, but it stopped immediately with observation. In any case, occlusion of the hepatic artery or hepatic infarction was not observed. Conclusion Transarterial fiducial marker placement appears to be a useful and safe procedure for proton therapy of malignant liver tumors.