A case of sclerosing encapsulating peritonitis (SEP) associated with liver cirrhosis (LC) and complicated by diffuse large B-cell lymphoma (DLBCL) is reported herein. A 49-year-old Japanese man had undergone peritoneo-venous shunt against refractory ascites due to hepatitis C virus-positive uncompensated LC for 2 years. After he received a diagnosis of DLBCL of the left neck lymph node 3 months before his death, palliative care was given because of his poor general condition. He developed severe abdominal distention and pain over 1 week and was found to have marked ascites and whole bowel lumped together on abdominal CT. At autopsy, the peritoneum was covered with a thick white membrane and the bowel could not be distinguished, which was macroscopically characterized by a cocoon-like appearance. Histology indicated a proliferation of diffusely thickened or hyalinized fibrocollagenous tissue in the entire peritoneum with a slight chronic inflammatory infiltrate and without remarkable change of mucosa. A diagnosis of SEP, also known as abdominal cocoon, was established based on these features. Additionally, in the abdominal cavity, a large amount of serous ascites and multiple peritoneal nodules or masses involved by DLBCL were recognized. To the authors' knowledge this is the first case report of SEP associated with LC and complicated by the invasion of DLBCL in the abdominal cavity.