Inflammatory bowel disease (IBD) with a subsequent diagnosis of non-Hodgkin lymphoma has been reported. Gastrointestinal tract involvement in mantle cell lymphoma (MCL) is rare and many case reports of MCL simulating IBD have been reported. Lymphomas have been reported in associations with chronic ulcerative colitis (UC). The prognosis of lymphoma in patients with IBD is not well known. We report a case of MCL occurring in a patient with UC. This is a case report of a patient presented at Oncology Center Mansoura University. A 56-year-old male was referred for evaluation of thrombocytopenia and microcytic hypochromic anemia with a history of prior bleeding per rectum. Clinical examination revealed ecchymosis, mild splenomegaly, and small right submandibular lymph node. Colonoscopy showed diffuse mucosal erythema and edema of the transverse, descending, sigmoid colon, and rectum; ulcers showed loss of the vascular pattern. Extensive ulcerative colitis had a Mayo score 1. Multiple biopsies were taken and microscopic examination revealed mild focal active colitis with multiple hyperplastic lymphoid follicles and no evidence of cellular atypia in the examined material. Gastroenterologist started the patient on corticosteroids. Virology screening, ANA, anti-ds-DNA, and Helicobacter pylori stool antigen tests were negative. Thrombocytopenia was refractory to corticosteroids, and the patient suffered from recurrent episodes of melena; the patient was shifted to eltrombopag. For the UC, mesalazine 500mg bid and proton pump inhibitors were prescribed. On further evaluation bone marrow examination revealed infiltration with small B-cell lymphomatous deposits (49%) positive for CD20, CD5 and Cyclin D1: MCL. Lymph node biopsy was also consistent with MCL. A suspicion that IBD could be an extranodal lymphomatous infiltration was raised; revision of the GIT pathology was consistent with UC and no malignancy was detected. Our patient received R-CHOP/DHAP protocol and achieved complete remission. The patient remains in complete remission on rituximab maintenance. The patient is ongoing follow up with gastroenterology for management of abdominal pain and gastrointestinal bleeding. We must pay special attention for lymphomas occurring in IBD; exclusion of extranodal infiltration is mandatory for patient management.