Abstract Introduction/Objective Lymphomatosis cerebri (LC) is a term used to describe primary central nervous system lymphomas which may present as diffuse, non-enhancing, infiltrative lesions. Although most of these are diffuse large B-cell lymphomas, there are other cases in the literature of low-grade B-cell lymphoma, Burkitt lymphoma and T-cell lymphoma. Methods We present a 76-year-old male with a past medical history of chronic lymphocytic leukemia (2014), diffuse large B cell lymphoma of right jaw (2019) s/p chemotherapy, prostate cancer s/p prostatectomy and radiation. The patient presented 3 weeks prior to admission due to generalized weakness with rapidly declining mental status, anorexia and deconditioning. MRI brain revealed scattered T2/FLAIR signal abnormalities in the cerebrum and cerebellum. An assessment of encephalitis was rendered. The patient continued to worsen and passed away. Results On autopsy of the brain, there was extensive involvement of the basal ganglia, cerebellum and all cerebral lobes by a lymphoid process. It had a predominant perivascular, parenchymal and leptomeningeal distribution. Microscopically, there was a diffuse proliferation of small to intermediate sized lymphocytes with mature chromatin. These lymphocytes were neoplastic B cells which stained for CD20, CD43 and negative for CD5, CD3 and CD23. It also showed low Ki67 index. This immunophenotype is most consistent with a marginal zone lymphoma. Conclusion It is quite rare that our patient’s infiltrative process was a small mature B-cell lymphoma most compatible with marginal zone B-cell lymphoma. Our case illustrates the importance of recognizing this entity, particularly in context with different morphology since our patient had three different types of lymphoma. The dilemma in diagnosing such lesions can lead to delay in diagnosis and institution of appropriate management. Thus, knowledge about its imaging and morphological features is very critical for correct categorization and to avoid potential misdiagnosis.