Mucosa-associated lymphoid tissue (MALT) lymphoma is one of the most common lymphomas and accounts for about 7% of all newly diagnosed non-Hodgkin's lymphoma (NHL). The clinical course of MALT lymphoma is relatively indolent and, in the majority of cases (50%), the lymphoma arises within the stomach. Primary central nervous system lymphoma (PCNSL), an uncommon variant of extranodal NHL, can affect any part of the neuraxis, including the eyes, brain, leptomeninges, or spinal cord. Herein, we present a rare case of PCNSL, which occurred one year after radiochemotherapy of gastric MALT lymphoma. A 62-year-old man presented with a 3-day history of left facial palsy. One year ago, he underwent antibiotic eradication therapy of Helicobacter pylori, local stomach fractional radiotherapy, and chemotherapy for gastric MALT lymphoma. Magnetic resonance imaging revealed a strong enhancing solid mass in the right frontal lobe. The tumor was completely removed, and the histological diagnosis of PCNSL developing from diffuse large B-cell lymphoma was made. Although elucidating the correlation between the first gastric MALT lymphoma and the second PCNSL seemed difficult, we have postulated and discussed some possible pathogeneses, together with a review of literature.