Mesenteric fibromatosis is a benign locally-aggressive mesenchymal neoplasm that lacks the potential for metastasis. It is related to Gardner's Syndrome, previous trauma, abdominal surgery, and prolonged intake of oestrogen. Differentially diagnosing this from similar tumours is crucial in order for establishing the appropriate treatment and only immunohistochemical features can be used for a definitive diagnosis. Although medical therapies play a role in the treatment of mesenteric fibromatosis, surgical resection is the gold-standard procedure. Our case study is a 40-year-old male with a concomitant diagnosis of non-Hodgkin lymphoma and mesenteric fibromatosis, not associated with any of the risk factors mentioned above. We performed CT and PET scans and observed a vascularised and well-defined mesenteric centre-abdominal hypermetabolic solid mass in contact with the gastric body, duodenum, body and tail of the pancreas, transverse colon, and spleen. An ultrasound-guided tru-cut biopsy revealed features suggestive of mesenteric fibromatosis. An elective laparotomy was carried out and a giant mass, arising from mesentery, was excised, including a partial gastrectomy and segmental resection of the transverse colon. Distal pancreatectomy, small bowel resection and successive splenectomy were performed due to a large hypertensive component. The postoperative period was uneventful. The histopathology of the surgical pieces was compatible with intra-abdominal desmoid fibromatosis. As far as we know from the literature, this is the largest mesenteric fibromatosis tumour ever to be excised. We also noticed that this is the first reported case of the concomitant presence of mesenteric fibromatosis and non-Hodgkin lymphoma that is not related to any of the described risk factors. Further research is needed to establish what type of association this presentation may indicate.