Diffuse large B-cell lymphomas (DLBCL) are the most common lymphomas worldwide. They also appear to be the most common primary retroperitoneal lymphomas, but this presentation is relatively uncommon in the literature. Retroperitoneal masses, including lymphomas, often present with nonspecific symptoms and laboratory values, necessitating radiographic assessment and consideration of mass biopsy prior to the initiation of treatment. Here we present a case of a primary retroperitoneal DLBCL as well as a review of the clinical presentation, imaging findings, and differential diagnosis of such tumors.