The treatment of B-cell non-Hodgkin lymphoma, the most common posttransplant lymphoproliferative disorder, is not well defined. Herein we have reported a case of gastric mucosa-associated lymphoid tissue (MALT) lymphoma with rapid, persistent, and complete remission after conversion of the immunosuppression from cyclosporine (CsA) to sirolimus (SRL). A 42-year-old woman underwent renal transplantation in 1992 with no major abnormalities until 2006 when a gastroscopy performed to investigate dyspeptic symptoms showed a mixed MALT gastric lymphoma (with low- and high-grade components) associated with the presence of Helicobacter pylori infection. Two therapeutic interventions in a 1-week interval were performed: treatment of the H. pylori infection (omeprazole, amoxicillin, and clarithromycin for 14 days) and modification of the immunosuppression by substitution of CsA and azathioprine (AZA) with SRL. Control endoscopy performed 1 month later showed persistence of H. pylori infection and absence of the gastric tumor. New endoscopies performed at 2 and 7 months after therapy confirmed the absence of neoplasia and H. pylori eradication. Currently, the patient has no complaints, displaying a creatinine value of 1.8 mg/dL and a hemoglobin of 9.4 mg/dL using SRL and ibersatan. SRL has been studied extensively as an anticancer drug, acting as a mammalian target for rapamycin (mTOR) inhibitor. Accumulating data support the role of mTOR in lymphomagenesis. In conclusion, our case of gastric MALT lymphoma in a renal transplant patient displayed a complete remission after alteration of the immunosuppressive scheme with the introduction of SRL.